I am enjoying sex with my girlfriend, but having a tough time reaching climax....?

I think it is all in my head, and it is really busting up my ego. I feel like it is snowballing, and it is totally messing up sex with her. Any advice, what the hell can I do??? I'm NOT gay and I'm definitely falling in love with her, I just don't want our physical life to become an issue. We are both 21.