I would have thought that you should be looking at her feet and how she is shod first.
Does your farrier cut her toes back or leave them long? If they are left long then this could be causing her to trip. Otherwise she may be too much on her forehand and not working properly (being a bit lazy).
No her feet are fine now as had them corrected as she was very boxy when I got her, She does carry fat over her shoulders which is very slowly coming off. And her back has been rechecked recently and is all fine.
Do you need a hanging cheek? Not suggesting for one moment that you haven't thought about it, but have you tried just a loose ring snaffle, or a Sprenger training snaffle?
Doesn't sound like she needs a strong bit, so just a thought