Gosh, I've had this problem as well. And still do honestly.
When I first got my greenie back from the trainer I rode him for a few weeks and then he had a major spook where a crow flew up off the ground and he spun and bolted, bucking as he went. I came off and landed hard. Nothing broken but I had bad back pain for about two weeks and couldn't ride until I got a steroid shot in my back. I think that two weeks let the fear sink in. I've had fear issues riding him ever since.
Sometimes just thinking about riding him gives me butterflies in my stomach. But if I ride him regularly, then I have confidence and am not afraid. But if we have bad weather for a week and then I go to ride him again, it's almost like I am starting all over.
Now I did come off him another time and landed softly and that didn't seem to stick with me as much as the first fall. I think the person who said it was PTSD is right. Because it's almost like I am having some sort of panic attack. And nobody around me really understands. They just think I need to "toughen up." And I do. But I don't think they realize how bad fear is.
One time I got on my horse and everything started going wrong......in my head anyway. Dogs were barking, horses in the pasture we were riding past were galloping and bucking, crows were flying around. I had a total crying meltdown right there on the horse. All I wanted to do was survive the ride. But you know what? The horse, whom is only 3.5 yrs old, just stood there while I had a meltdown and didn't react to anything. Not the horses bucking, not the crows flying, not the excited barking dogs. Not even me turning into a sobbing mess. He just stood there. I ended up forcing myself to ride out like I had planned and after about 1/2 hour I regained my confidence and enjoyed the ride. I knew if I quit when I was afraid I might not have the guts to ride him again. And it was the right decision. He's a good horse. Green, but he's got a good brain on his shoulders most of the time.
Gradually I am getting better and more confident but it's been really hard.
There was a time I was pretty carefree on a horse. It's funny how one fall can shake your confidence to the core. I still have anxiety and relapses of fear, but I want to get back to the place I was before so that's what I'm working on.
I don't know anyway to get over it other than to try to replace the bad memories with good ones.