Delayed Ejaculation - Interfering with my Sex Life

I'm 21, I've had four girlfriends since the age of 17, and delayed ejaculation has always been a problem. I don't know why I suffer from it. I guess I just can't relax during intercourse. I've read from many sources that it might be because the man is not aroused, but believe me I am. My current girlfriend turns me on a lot. I just feel very disappointed, for her sake, that I can't reach climax.

I can masturbate next to her and ejaculate on her body which she seems to like but it's not really the same. We've recorded some "home movies" and I can get off and ejaculate extremely quickly watching these. I just can't do it during intercourse, or if she's performing foreplay. It probably makes her feel as if she's not pleasing me but I do assure her that this isn't the case. I just can't please myself, that's all. I'm always giving her foreplay and making sure she is enjoying herself (fortunately I can fairly easily make her reach climax through foreplay), so maybe I am having trouble letting go of such responsibilities.

Thought I'd also mention that whilst I can ejaculate next to her I have to do it much more vigorously and I find it very difficult to do it if she's looking at me. :/

Any ideas?