I agree with findeight above. I strongly suggest finding help, even if you have to haul her to someone. Reading their body language, staying 100% aware and tuned in at all times, and knowing exactly when she’s thinking of or starting to bolt is all about timing and is easiest and most effectively taught in person. I know you say there is no one within an hour of you, but unless you live literally in the middle of the mountains or somewhere where there are no horse owners at all, I guarantee you can find someone experienced with youngsters that would be willing to help you.
Every time she gets away from you, you will have to do quite a few successful repetitions of her NOT getting away from you, to undo it. It sounds like it has now become a bad habit. And she is only going to get larger, and harder to control. Trying to correct this on your own can be dangerous if you don’t know how to handle it, and there is NOTHING wrong with asking for help. It’s those that refuse to ask for help that end up in a pickle, or injured. Or with a horse that no one can handle.
Ask around at your local feed store or tack shops, and ask for referrals. Usually, the good trainers will come up in more than one conversation with one person, and you can get some contacts that way. Good luck OP.