It’s definitely not an uncommon problem. I’ve never come across a “special sauce” that fixes it for everyone, but there are definitely things you can try that might help shift the balance of worry > desire to move up the other way. Without knowing what you’ve tried already, here are things that might benefit you:
Go watch people jump 4’ or some GP rounds. By the time you get back to your 3’ 3’’ jumps they’ll look teeny and totally unassuming.
Gymnastics til you’re blue in the face. Can’t screw up your distances when you’re trotting in and jumping out, so it’s a great way to just get used to looking at fences at 3’3’’ or higher and getting used to it as the new “normal”. When my trainer builds them up, she doesn’t even tell me when the height goes up and that makes it easier for me to not worry about.
I also remind myself to focus on my past successes. I’m retraining an OTTB and when he first arrived at the barn, he was all GO and zero whoa. He’s also a bit of a nervous horse to begin with and his way of going made him just feel like he was just one strong breeze away from bolting. I wanted to ride this horse, and I knew I was capable, but his freshness from the track did mess with my head a bit. I would remind myself that although he always felt like he was going to bolt, he never actually did, so I needed to trust that that would continue and ride confidently instead of letting the fear of something (that had never happened!!!) eat at my brain. And even if he did bolt - so what? I can ride a galloping horse. Even if he ran for miles and miles and I couldn’t get him to stop, I’m not really in any danger because I know I can stay on a galloping horse and he will stop eventually. So I’d encourage you to think about what it is that you’re really worried about, and consider if it’s actually something you have a real reason to worry about. Getting a bad distance? Maybe a dirty stop? Surely those things have happened at 3’ too, and you’re still here to tell the story.
I also make a very conscious decision to trust my trainer. When I’m worried, I remind myself 1) that I like my trainer 2) that she’s very experienced 3) that she knows my riding ability, strengths and weaknesses well and 4) that it’s in her best interest for me to NOT get injured. And with that kept in mind, if she tells me to do XYZ, then it’s because she is confident we can do it and I’m going to trust her expertise and give it a go.
… and to follow that up, in those situations I’m still anxious about it, despite trying to trust her. So I do the 3-2-1 rule. I count down from 3 and when I get to 1, I just DO THE DAMN THING. No thinking, no fretting, no trying to plan. Just. GO. You can analyze and plan next time, but I think the biggest thing is to just do it once.
And if you’ve tried all of this, and you’re still worried sick, then maybe reach out to a sports psychologist. They’re literally trained to help athletes with exactly these kinds of problems and can give you a lot of useful tools to help in these situations.