Hi, I'm 18 and when I'm with my girlfriend and we are cuddling or we are in foreplay (kissing, etc.) I have a great erection. But when we plan to have sex and I want to do it, we start kissing and I just can't get hard. I even went to the bathroom and tried to get myself going and I ended up ejaculating and I wasn't even all the way hard. I just feel I get nervous right before cause I keep thinking about past times when I couldn't get hard. This is so frustrating to me and I don't know what the problem is but I just want to please my gf cause I know she wants to have sex and I also do but it's just this keeps happening and I feel like she's getting over it and she's going to leave me. I stress myself out over these things and it gets to me. If anyone has any advice please give it to me.