I'm dealing with a similar loss of confidence.
I was rodeo bucked off another boarder's mare while trying to be nice and help the boarder try out her horse for the first time. I wouldn't have gotten up on the mare, except I was told that she was broke and had tons of rides on her, she just hadn't been ridden in a while. I climbed up on her after we weighted the saddle (and she didn't flinch) and as I was leaning down to catch my right stirrup, she crow hopped on me. No big deal, I've ridden worse. Apparently it pissed her off that I didn't come off because she pinned her ears, whipped her head around and full-on rodeo bucked me. I flipped over her right shoulder and landed on first my butt, then my back, then my head. First time in two years I wasn't wearing a helmet and I was seeing stars. According to my friend (who was in the round pen with us), the mare went nearly vertical when she bucked, came down on her side, rolled over the saddle, scrambled up, and then, without even pausing to shake off the dust, came after me with both front feet as I lay on the ground, dazed. Turns out the mare had NEVER had ANYONE on her back before I climbed up there, according to our BO.
I severely sprained my ankle (my left foot tangled in the stirrup as I came off), was in a splint for a week because the ER thought I hairline fractured my tibia (turned out to be the shadow of a bone spur that made it appear to be a hairline fracture), and in a walking cast for two weeks.
I thought I was going to be fine getting back up on another horse. I was bucked off (my fault) when I was younger and was back riding the next day. No problems. So, I eagerly climbed up on my greenbroke two-year-old who I've been waiting for two months to ride. All of a sudden, I was a nervous, shaking train wreck. If he flinched, I started to panic. Today, we went on a ride around the neighborhood (his first ride off-property) and, while he did great for a green-as-grass horse, I didn't do so well. I finally realized that I was not in a good state to ride after he spooked once (it wasn't even a real spook...he flinched when a dog started barking and trotted to catch up with the other horses), "bolted" (trotting) after the other horses when he thought he was being left behind, and flinched when my friend reprimanded her mare and her mare got all pissy, so I dismounted and walked the entire two miles home leading my horse. It totally wasn't him. It was me.
Before that mare threw me, is "spook-and-trot" that he did after the dog startled him would have been absolutely no big deal. Today, I actually shrieked and did everything wrong (sat forward, pulled up on the reins, shoved my toes down)...until the trainer (who was riding with us...can't call her "my" trainer, since all she did was ride my horse the first few times he was under saddle...I don't take lessons with her, though) called out "Sit back!" That snapped me back to reality and I was my old, calm self again and got him under control. It's like my brain completely shuts down when something "scary" (that isn't really scary) happens.
My friend, who boards at the same barn I do, doesn't get it. She's never been thrown from a horse and her confidence "issues" were all because she had her stirrups too long (she was extremely uncomfortable at the canter). The trainer looks at me like I'm a horrible rider and shouldn't be on a horse. But this nervous, scared, panicky person ISN'T me! I rode one of the dude string horses when I first started boarding at this barn (first time on a horse in almost a year) and we went on a trail ride. My stirrups were too long (couldn't shorten them, or they would have been too short), which made me uncomfortable, but I rode Reno's random trots to catch up to my friend's mare with no problems, rode his wading through a bush at random with only mild angst, and even rode out his MAJOR spook coming off the highway on our way back because a motorcyclist got too close and revved his engine and flashed his lights at us...all with absolutely no problems or fear.
Sorry...this got really long...but I'm just so frustrated and I really don't have anyone to talk to about it who is understanding and won't just say "Get over it."
Do not tell me I can't...because I will show
you that I can.