Agree Woodhaven but stock trailers are roomy enough the OP should be able to get out safely imo. I would be more concerned about her trying to bolt through a human door in front of her.
JW was this mare actually abused?
Yogiwick, good points but if the op ties the horse in a stock trailer and starts to walk out the back and the horse suddenly tries to pull back, they can throw themselves around quite a bit and smash into the person and I think this is a horse that could panic because of her previous accident.
As to the escape door, a horse could try to get out it as well, when I go in with a horse in a trailer with an escape door, I like to have the door unlached but still closed, then I tie the horse and I know if something happens, I can just push the door and step out. If the horse is tied she can't follow me (hopefully). In over 55 yrs of trailering, I have never had a horse attempt to exit through the escape door (partly because the door is never open as the horse goes into the trailer) but I know it is a possibility, but in this case I would like to see the op as safe as possible so this is why I suggest an escape door.
It's too bad that the op does not have a trailer so they can take their time and do some trailer exposure at a slower pace.
I should add here that I have seen horses that have been in trailering accidents, that load very well into a trailer after. For some reason they don't associate loading with the accident, but they might get nervous after or, and I hope this is the case, have no problems