Really good to have yet another older rider - And very sorry to hear of your fall. Well done for getting on the horse again!
Interesting point about mounting. You may like to know that the trainer Mark Rashid said that after the rider mounts, both horse and rider are naturally tense. Even the best riders. It is instinctive reaction so not something to blame yourself for. The horse is having to steady itself to the weight on its back and the rider no longer has his or her feet on firm ground.
Rashid's advice is that, after mounting, one should take a little time to relax in the saddler, breath deep and soften your hands on the reins till the horse lowers its head and softens too.
I find this very useful. One usually mounts in a busy yard and hurries to adjust the girth or stirrups - or the RI does this for one, eager to get on with the lesson. But there is real benefit in not rushing it.
The other suggestion is that after a major accident like you had, you should each time only do what you are comfortable with - even if you are only being led round in walk. Once you are comfortable doing that - then move on to the next stage. I have once or twice been really truly scared by serious accidents to people we knew and I got back to normal very slowly, only doing the riding I enjoyed. First in the school and then venturing out hacking on a very old slow horse with an old slow teacher too. There are moments when we older riders need to be encouraged to do more than we thought we could. But after an accident is not the time.
If it is the horse that is the problem - you can find plenty of advice for teaching a horse to stand to be mounted and most of that is done from the ground - so you can do it with a RS horse.