I think it could an issue of both. Confidence has a lot to do with performance. If you think you look crazy or don’t think you’re doing well your performance will reflect that. What helps with confidence is practice. Now for flexibility trying different positions can assist with that.
I definitely find that when I've been in the gym building my leg muscles, I perform WAY better on top. I have more energy, balance and control. BUT (and Ashley, please correct me if I'm totally off on this) I've found that guys often go a little limp when I'm on top. After talking to a few (over years lol) I've heard back that they have to try harder not to cum. I'm on top, gravity's got my wetness coming down, it can end quickly lol. It's worth having a convo with your partner to ask how they feel when you're on top! Cuz feeling like he's not responding can mess with your head, too. Meanwhile he might actually be TRYING not to "respond" :)
How do I get better Or figure out what the problem is?
Bike riding, jogging, stretching. Deep squats. Hamstring stretches.