Trained right, most horses will work great in a snaffle. I personally don't ride in anything but a french link loose ring, and I've ridden or trained arabians, quarter horses, tbs, warmbloods, saddlebreds, all who work great in that bit. I have only started one horse in a bosal, every other horse was started in a snaffle, you just have to be careful about not pulling on her mouth excessively with a bit. I have known of 14+ hand haflingers, as well as Mustangs that are 15+ hands, so it is possible to have a 15.2 hand Mustang Haflinger cross. Size does not matter when it comes to jumping ability. I know of a horse that went to the olympics one year that was a 14-14.2 hand (can't remember the exact size) I believe arabian for show jumping, so it is possible for a small horse to be able to jump big. I do think that you need to do a lot more ground work before getting on her again, enlist the help of a trainer, and ask about, and or look up ground driving, as I have had wonderful success especially with the more stubborn horses if I do ground driving and get that down well, before getting on. Good luck, hope that she gets better.