You said earlier that the accident wasn't your mare's fault? Just keep reminding yourself that. I have some degree of fear before almost every ride. I have to remind myself that my boy has never done anything to make me mistrust him. A couple weeks ago I had a day when I didn't think I could even get on. Usually after about 10 minutes of riding I'm A-ok. You're not alone in being fearful, a lot of adult riders go through this.
Remind yourself how many good rides v. bad rides you've had. I actually figured out the good ride v. bad rides stats to put it in perspective for myself. Out of about 252 rides about 4 were bad. One of those was totally my fault, 2 were spooks which I can't blame him for (I would run at a large falling branch, too!!), and only 1 was him being naughty. So how about your horse? Think about all the rides that went right and how good your mare was for those.
I also have a trainer who is very understanding of my issues. I was feeling anxious at my last lesson, so she found exercises we could work on at the walk. So I would try to search out a trainer like that.
I agree with everyone else, take your time! Riding for five minutes is better than not getting on at all. Do you have a round pen you can just putter around in for ten minutes a few times a week when you're up to it? Maybe just do ground work for a while?
I would think long and hard about giving up my horse. Personally, even if I didn't ride anymore, I would keep my horses. I get a lot out of just being around them. I call horses "nature's xanex". From the ground, anyway!