Hi & welcome OP!
Wrote most of this this morn, when there was only Blue's response, now coming back to it there are lots more posts to read!
'Baby steps' is the way to go I reckon. Same as you'd desensitise a horse to 'scary' stuff. If you try to force yourself to do too much, you're 'practicing' that fear association & also making for more chances of things(even little things) to go wrong, which further reinforce your emotional reactions. You need to practice feeling relaxed & confident about it instead, which means never pushing yourself too far, too fast.
I went from breaking colts, riding stallions and galloping ottb's down winding trails fearlessly, to barely being able to make myself get on. I would tack up, force myself on, walk a few steps, get off before having a complete panic attack while shaking like a leaf, untack and go home. I am now back to starting colts and riding ottb's fairly fearlessly, but a bit more cautious..
Oh Blue, that sounds pretty similar to my experience. Except that when I was at that stage I also had a very young family & little time, a green horse... that I was 'rubbing off on' badly(if I expected it, he was going to deliver!), and no one to even ride with. I kept trying to push it, because, well, I felt stupid for feeling like that, when I'd been doing it for years & I 'should' be capable. Got so bad that my heart would start racing even at the thought of throwing a leg over!
I trained horses & other animals for years, I have taught kids to ride & be confident, but duh! I had forgotten to treat myself in the same way - start wherever the horse/child is at, and work gradually from there, building on a foundation of confidence with 'easy' things.
So... I started myself back at 'kindergarten' with no intention of even sitting on my horse yet. We worked at both getting really confident about being in position at the mounting block & me leaning over him first. Only when we could do that without a worry did I start throwing a leg over... & I even started that with the other leg firmly planted on the mounting block! When I first came to sit on him, it was fleeting & I slid off the other side or back on the block before either of us got 'tight'. Then I stuck to a small arena size area & just sat on him & walked around a bit... you get the picture. It took a while, but I reckon I'm back to where I was before the... breakdown
I'm just not sure of my reactions these days to other people's horsey accidents... namely my kids! Used to think of myself as level headed in an emergency, but when it's your kids...