advice, intimacy problem maybe?

Whenever I am close to anyone intimately I immediately (when we start doing whatever) feel not turned on, even if i was in in anticipation. It's very frustrating, and so I usually just try to make it about them, because no matter how hard I try, I just can't feel anything for myself. When I'm alone, however, I'm perfectly fine and can do all the things you're supposed to do, and am in fact a very ual person. I don't know why I'm just immediately switched off when other people enter the equation. Has anyone here felt like this? I would love some advice, anything is appreciated.