Go back to basics. Day one tack her up. Day two tack her up and lead her with the tack on. Day three tack her up and lead her around in a bigger but comfortable space. Day four tack her up and walk her past the mounting block. Day five tack her up and ask her to stand at the mounting block. Day six get a helper and have them hold her, jump up and down beside her for a while. If she is extremely comfortable you may put weight in the saddle. Day seven leadher up to the mounting block and lean over her so you are completely lying on your stomach(but so that if she does anything you land on your feet not your head) get your helper to lead her around so she gets used to feeling weight again. Day eight try whilst on her the same as yesterday patting her bum and desensitising her to that feeling. The quietly put your leg over. If she tenses up hold your leg there and slowly back of until she relaxes. Do this until you can put your foot in the other stirrup. Use your helper another 2 times before you ride her on your own again. Good luck
I would first have her checked by a vet sounds like the bad behavior could be coming from a painful back or saddle. After you check that out and everything is ok make sure you use a mounting block. But first I would just start by teaching her to stand quietly by it then I would climb up on it with out trying to mount and keep slowly moving forward until she is completely comfortable with it. Good luck and be safe, it would be wise to have a professional help you if re working the basics does not help. You could also try lounging her for 20min before trying to mount she might get her jitters out.
Go right back to basics and take small steps. I'm not sure if you have ever backed a horse before, but if you haven't, it might be worth getting someone who has experience backing horses because by the sounds of it, the horse needs the same reassurances that a young breaker needs.
At what point does the horse start to worry? Is it when she sees the saddle coming? Is it when you line her up to a mounting block? Is it when you stand on the mounting block? Or when you put your foot in the stirrup? There's a small chance that the horse is completely calm about the whole situation until the exact moment your leg is parallel with her back, but it's much more likely she starts getting concerned well before it. You need to figure out when this point is (look for little cues like a tightening of the eye or nostril, or looking away from you/the saddle/the mounting block etc) and work from there.
At the moment, your horse has a negative association with being mounted which you can identify the source of. What you are aiming to do is re-create the positive association. Find the point at which your horse starts to get uncomfortable and begin to give treats or scratches. Don't move to the next step until the horse stays totally relaxed at that point despite doing it repeatedly, and then do the same with the next point. This could (and likely will) take weeks before you are actually sitting on her, but the goal is to stop the association of being mounted with the accident, and start the association of being mounted with something good.
You haven't mentioned the nature of the accident, but it's absolutely worth getting your horse checked over the make sure there's nothing going on that causes her pain at the point in which you are swinging your leg over. There's a lot of movement and shifting of weight at that point and it could be setting something off.
It's might also be worth checking if this happens all the time, or if it's only when she's in the arena, or has a saddle on etc,.