I think your best bet would be to find a pro trainer/instructor to help you out here. At least another experienced horsey person to help and consult. If that isn't possible, I think I'd basically re-start her. Do lots of groundwork
, get her quiet, willing, and respectful in hand. You can do "trail walks" with her, make obstacle courses to go through together, etc. Build rapport on the ground. When she's totally accepting and obedient on the ground, and you are comfortable, try getting on in some kind of enclosure. A roundpen or arena would help, but I used my turnout area as a surrogate arena for years before I got my arena. STAY AT A WALK. Get her totally obedient, soft, and willing at the walk. Talk to her, rub her withers/rump and keep both of you relaxed. Keep a loose rein, make it her responsibility to stay slow (without you reminding her constantly). If she speeds up, bring her into a circle (a pretty one, there's no reason to not have her thinking
) and calm her back to a walk, then give her her head. You can build some gaming patterns if you like, and walk them. Lots of walk/stop transitions, build a solid whoa button. "Whoa" should be the strongest word in her vocabulary. Just give her a job and keep her interested. Use your imagination here. When she masters sanity at the walk, try the trot the same way. When her trot is golden, try the lope. If you take her on a trail ride, stay at the walk and stay with another horse (preferably a seasoned, quiet trail horse), if possible. The company may help her stay calmer, and she'll probably be less likely to bolt from the "herd."
This is really the Readers Digest version, but I hope it helps you a bit. Please, if at all possible, get some experienced help with this.
Also, depending on her build, weight may be the issue, even bareback. 12.2 isn't very big at all. Do you have a saddle that fits her? You can saddle her and then add weights in small incraments to see how she does with lesser weights to test this, but, again, please get some help. Do your best to rule out any physical reason for her behavior.
I hope that was helpful, feel free to ask any questions. Good luck, and be careful!