Hi Marsh, welcome!
I felt like I was reading about my current mare for half of your post. I had the exact problem when I first got her except that when I could eventually stop her after I got on (key word being eventually) she would rear.
I can't speak for your girl Flash but my girl Bobbie was in a complete panic and so scared when I got on that when she eventually stood still, her whole body would shake. I could go in to what happened to her at the track, but suffice to say she did not have a very enjoyable experience there. I wonder if your girl is similar.
So, first and foremost you have to be a strong leader. If you are dealing with a frightened horse or even just an unsure
horse, they are going to look to YOU for leadership and you have to provide it in a cool, calm and collected manner, no matter what they throw at you.
Secondly, even if your girl DID have a terrible time at the track, that is behind her now. You can't treat her with kid gloves (not suggesting you are) in an effort to make up for the tough time she has already had. Treat her with kindness of course, and fairness, but if you need to be firm then you must be firm. Bobbie (my OTTB) had two 'come to Jesus' meetings in the first few months that I had her because she needed to know that some behaviours are completely unacceptable. The first was for rearing, the second was for kicking. Neither have been an issue since.
Thirdly, for the mounting at the block issue *sigh* I remember how hard that was! Like trying to jump on a moving freight train from a small box. Take a look at this vid that one of our members made, you might find it helpful: Ever have trouble mounting at a block?
As for the bolting while you are mounting, I did this:
Do you check her back to the side when you get on? It's not restraint, it just prevents her from flat bolting or bucking when you mount. Just shorter your left rein so that she tips her nose to you and hold that steady as you mount. That should at least help given the problem as you've described it.
Your description of how you get on made me smile, I can imagine exactly how it is happening
Good luck, she sounds like a wonderful horse! In case you were wondering, it took me a couple of weeks to iron out my girl's rearing and a couple of months to sort out the mounting. She still walks off occasionally but the bolting has stopped completely. You'll get there!
ETA: Pain disclaimer. I had Bobbie vetted and seen by a chiro in the beginning to addresss the pain issue, you may want to look in to doing the same.