I've been going out with this girl for almost six months and we are in love. But everytime we try to have sex I just can't keep it hard. I get really hard during foreplay but everytime we try to actually have sex it just goes down again. I havn't had sex before and at first it didn't worry me cos I thought it was just nerves but it's happened three times now. What else can we do to enhance the mood or whatever to help me?