How to cum during sex or sexual interaction

Sophie

Guy, I have an issue.

I can cum on my own normally whilst laying down, and concentrating really hard on cumming

But I have never done it with a partner before. Ive felt it cumming, but then it goes away. I also can only really do it laying down. Does anyone know how to help me out with this issue, what I can do to train myself to be able to cum in different positions etc!