Can I give a bit of unsolicited training advice? In my opinion:
Don't canter under saddle until she is completely comfortable and balanced at walk/trot under saddle and balanced cantering on the lunge/free lunging. Don't ride much- I would do a lot of ground work, personally, and maybe ride two-three times a week, lightly, until she's settled and comfortable.
Don't let the tie-down tempt you. It will only remind her of bad things. Pitch it... it is a fix of a symptom... fix *the problem*- probably lack of balance, strength, and trust; also possibly hind end lameness (hoof issues, sacroiliac joint issues come to mind).
If you feel safe doing it- what about a rope halter/sidepull? Jumping hackamore? Eliminating the bit altogether for a little while may let her past some of her 'baggage'. If you do go into a bit- I would suggest a double-jointed 'bean' mouthpiece, dee ring or eggbutt snaffle. Not super thick (thicker is not always better), but not particularly thin either. Perhaps something like this:
http://www.doversaddlery.com/jp-korsteel-oval-mouth-eggbutt-horse-bit/p/X1-010063/cn/1583/
Myler bits are also rather nice in a lot of ways: I like the MB02 mouthpiece with an eggbutt. They are a tad thin though, so aren't always the greatest choice.
She looks good but still something up in the last pic. See how she's just not 'there' in her hind leg? Could just be a wonky photo, happens often enough, but something to think about.
Good luck- these horses can be tricky but worth every bit.
