Why cant I maintain an erection during foreplay?

I have no problem with keeping it during intercourse or even during oral while i'm getting constant stimulation but during foreplay it's gets hard at the beginning and then just starts to go away. A while ago during foreplay it happened and now it seems to be a recurring problem. I don't think there is anything physically wrong with me as I'm only 28. I think now that it happened I start to think about it happening again and it's all in my head. How can I make this problem go away because my g/f is upset thinking it's her fault that she doesn't turn me which is very untrue because she's very beautiful and sexy and I love everything that she's doing. I just can't keep my erection. Please help.