OP-I have not read this all in depth, nor do I know what happened to you in your past. We all have issues. Certainly it is your choice if you decide to keep her. But, I would agree you are not doing her any favors, as she does need a job, and that job will help to insure that heaven forbid, something happens to you, she can go on in her job and not come to an end you would not want. Yeah, they can be trained older, or after being not ridden for years, but not a lot of folks do that. It is much easier to rehome a horse who is useable. Just a fact.
Anyway, as someone who has also had issues which led to a total lack of confidence, which led to me being "overhorsed" a couple of times, both of whom went on to others, retrained and have awesome jobs they excel at.
I have no idea how old you are, but I finally came to the following conclusions:
-I have horses so that I can enjoy them
-I do enjoy riding them
-I am NOT getting any younger
-I needed a horse I could trust
-I am a total wimp
-ALL horses have vices. You need to know what they are and make sure they are something you are willing and able to deal with. In my case, if one of mine EVER reared like that-I would NEVER trust it again. That is just me. I would still do everything I could to figure it out, but I would have to get rid of it. I have to know that no matter what, mine will not do anything dangerous. Period. THere are many good horses out there, all of them have love to give. Do not feel badly if she doesn't work for you.
Oh-and I would suggest a chiro in addition to the vet. Sometimes they see things the vet may not. Then you will know you have all the info, and can make an informed decision. Like I said-after a rear like you described-I would be GIVING the horse away with full disclosure to someone who can deal with her, and chalk it up to experience.