Do you work with an instructor at all? Sometimes, you just need someone on the ground to get you to do things you don't really want to do, but are capable of doing. It's a common problem that you have and it can be undone! You just need to step out of your comfort zone a little bit, and I find someone teaching you can help you to do that.
I totally agree with @abbyloveshorses you just have to think of how it will benefit you and your horse. I go on trail rides at a different barn sometimes in the summer on a horse that I don’t know at all. I get nervous when we start the trail ride but then I think “what is the worst that could happen? Fall off? Done that before.” This helps me a lot. When we start cantering I got nervous because we were in an open field cantering with a horse I didn’t know! But then after the ride I was disappointed in myself for not having fun and enjoying it. I made myself a promise that I would have fun riding even if I am scared to do it. Horseback riding is all about the experience and if I’m always scared then there is no experience! So basically I just ask myself “what’s the worst that could happen” and then try to just let go and have fun. Good luck!
I get like that sometimes, but then I think to myself what would happen if I didn’t do it? Would my next ride be worse? Would I have a lot of fun and want to do it again? Would I regret it if I don’t do it? Would she love it and discover her new favorite thing to do? Would this one thing help her calm down? Would this one thing make her improve? How will this one thing effect both of us? Will it help us or bring us down? Or could it be the thing I decide to do with her? Could this one little thing change both of our lives for the better? Once I answer most of those questions to myself truthfully, I make a decision, and I usually decide to do it. I also sometimes just say to myself, “YOLO!” And that gets me to do it😂.