Nicole,
I completely understand your struggles, as I have one child so far that is the same way (sweet, sweet spirit and utterly distractible).
I have spent my life redirecting this very bright, very animated, very wanting to please child, and the good news is that I have seen improvement through the years.
While I know it is incredibly tempting to downsize your older student to combine with your child in Bigger Hearts, in my heart I feel this would be a decision for the moment that doesn't truly address the core issues in the long run. While it would be easier to keep your child on task if you were overseeing her every moment, it still would not train her to oversee herself, manage her own time, or do what you ask when she is not in your sight. These are the core issues we all struggle with as our children mature, but it is the training in these areas that will pay dividends for your child in the long run. Another reason I'm thinking that the switch to Bigger would not be best is that it is likely not going to come anywhere near meeting your daughter's academic needs, as she was able to complete Creation to Christ last year. The step back to Bigger would be a huge backtracking for her in the academic arena.
I know you are likely weary, as I am many days too.
I honestly think the character training and responsibilty of raising our kiddos for the Lord wears me out more than the academics!
With your sweet daughter's needs in mind, I'm thinking a pull back to half-speed RTR is your best bet. I would shift your youngers all to a -4 day week for anyone doing a full-speed guide for the year (schooling only 4 days a week and carrying their current guides over to the next year). Doing a 4 day week helps all of us stay more focused and forces me not to skip or downsize, because I know I'm already only schooling 4 days a week! I love having Fridays off to make for a 3 day weekend each week. The kiddos have loved it too when we've done it in years past.
Next, I would sit down with your daughter and tell her that for the next week you want her to do only what the guide says and only what you tell her to do in any subjects you are doing outside of the guide. I would explain to her that it is actually disobedient when she works ahead, or reads ahead, because she is not doing what you ask, but rather what she chooses. She is, in essence, showing by her actions that her will is more important than your command. I would pray with her about it, and set up another meeting at the end of the week to review her progress. We have had to do this with our own boys, so you are not alone in this!
During the week, keep her nearby you, so you can see what she is doing. Try not to continually interrupt her flow of thought to remind her to get working. (I only share this thought, because I can get into a habit of nagging, which really makes me feel down and is so hard on the spirit of my child too. Perhaps you do not have this problem, but I mention it just in case.) Instead, have her come and show you each subject/box as she completes it as a check-up. Explain to her that when she proves to you that she can be trusted to do as you direct, she will no longer have to show you each subject/box as she completes it.
If she has been working a long time without coming to show you anything, simply ask her to come and show you what she has done. If she hasn't accomplished much, ask her if she's had some problems with something in the assignment. If there were no problems, then ask her what is taking so long? In this process, we're working to get the child to recognize he/she is dawdling, rather than the parent continually have to remind the child to get busy.
Explain that trust to be able to work on one's own is earned.
Keep up your subject/box checks, moving toward spacing out the tasks by checking every two boxes at a time, then three boxes at a time etc. as she shows fruit in her obedience and ability to stay on task. Keep up the weekly meeting on Fridays as long as you feel it is needed to talk over her progress. It takes 30 days to create a new habit, so keep your meetings at least 4 Fridays. As with anything, if both of your are diligent, you will see change.
My struggle always comes with being diligent for 30 days!
As you focus on your oldest child, you may consider dropping your other kiddos to half-speed for awhile too... at least for the month of rigorous training in accountability for your older, I would likely drop your 6 year old to half-speed in LHFHG (doing left side one day and right side the next). You could do the same for your child in Bigger Hearts with left side one day and right side the next day for a month. After that I would think about bumping up only the child, or children, who seem most ready for full speed. You may even have to keep your younger child or even children at half-speed all year to take the needed time to train your older child. But, it will be so worth it in years to come. I have seen a tremendous change in my son, even though we still have a ways to go!
This is something we have had to do with each of our sons. Most children need to be trained to be accountable and need to understand that when they do something different than what you tell them to do, it is in fact disobedient (even if the thing they chose to do was good). Kiddos need to learn to bend their will toward ours first, and then do what they desire after they obey first.
This is not a quick process. It is one that takes time and patience, but it does eventually turn the constant reminding and redirecting the parent has been doing for years into responsibility and accountability for the child instead. It is something many kiddos have never learned and is (I think) actually easier to train the child in doing when he/she is younger.
I once read an article by Dr. Dobson that said if you do not have your child's first time obedience (and also your child's heart which comes with obedience) by the time he/she is 13, you will likely never have it. If our children cannot be obedient to us than they will struggle with obedience to God. My husband and I were very convicted of this after hearing it and worked hard to make this a priority for our sons. While I do not succeed every day, I keep it ever in my mind knowing that if I do not have time to train them now I will never get the chance back. Be encouraged that you are not alone, and each of us must walk this path. It is always easier to keep our kiddos under our wings and at our side, but as they head into middle school and high school, it is truly best to gradually encourage independence through accountability. Otherwise, kiddos may find themselves dumped into the deep end of the pool expectation-wise when they hit either high school or college.
This is much more information than you asked for, but it is an area that is near and dear to my heart. Each year I try to prioritize which of my children need me most in any capacity for the upcoming year. As I listen to your post, I am feeling like your oldest child needs you most to train her in accountability and first time obedience this year. It is an important window with her from Dr. Dobson's perspective. It makes sense to me when I think of my children too!
This may mean that the youngers have a less mom-intensive year to allow you to focus on character training. This is fine from my perspective for this year, as likely next year a different child will be the priority and focus. It happens this way at our house.
Plus, as you train your older child this will make a huge difference in the time you have available the next year, and each year that follows, for your younger children coming up.
Anyway, just some thoughts to ponder. You may actually go a different direction than what I am mentioning above, and you will know your children best. But, I did want to give a long-term picture to think on as you ponder your path.
Blessings,
Carrie
. 11dd is using RTR, 8dd is continuing BHFHG which we started at the end of year and 6dd is continuing LHFHG. At this moment, 8dd probably gets most of my time, which is totally fair. She is firming up her reading skills and working on lots of great skills in Bigger. For the most part she is a hard worker and stays on task with some reminding. Although, she does try to slip some half hearted work, she really does pretty well. My 6dd does pretty well, gets off task at times and loves to play, but I am pretty satisfied with her work for her age. My biggest struggle is with 11dd, not really her, just school takes forever. This morning we got up together at 7 and worked about an hour before the others got up. She took 8-9 for breakfast, getting ready and room chores. From 9-5:30 she worked on school. I will admit, she gets a little dreamy headed during math, but even without math, school still would have taken way too long for me. The only thing I have added to her day is 15 min of spanish and a math drill. We are on unit 4. She did do Creation last year and some days would be long, but I feel like we haven't had one good one yet 
. I do have different times throughout the morning that we check in together and I give her instruction. I have used a timer giving her the max time suggested, but she can't seem to make it. Anyone else in these shoes? Anyone have any suggestions? I was wondering if anyone streamlines the instructions so it is not sooooo much. I hate to drop what we are doing; I really love the content of HOD. I just can't take these super long days everyday. I have other things I need to get to before dinner 
I loved math, and was in the top math group in my class with one other student, but not fast enough for honors. I still struggle constantly with time management, and always feel so behind on everything. Agghhh!! I'm still working on training myself! Unfortunately, I married a man who is not super efficient with time, either, so of course we have kids who struggle. It pains me to watch them, especially my oldest with math. All this to say, I can see why we may have a difficult time getting through daily lessons, and I am very open to any advice on how to train my boys to focus and use their time more efficiently. I am also loving HOD otherwise, and definitely wanting to stick with it. 