17 and shamefully have to admit to having ual problems?

Hey guys. This is really degrading but whenever I'm with my girlfriend, I can't seem to stay hard for . We've been to the point that I get the condom on and then I just lose it completely. When we fool around and she's playing with it either via her hands or mouth I get harder but it's never to the point that I would say it's "rock" hard as is necessary for . It's really embarring...The first time it happened I felt rushed and everything because it was spur of the moment and my brother ended up knocking on my door which saved me, but then last week we had a sleepover since my parents were upstate visiting my aunt and my new baby cousin and we fooled around for about an hour, then when we went to have I completely lost my . Not like it got limper, I completely lost it. I blamed it on the fact that my brother was walking around and I was nervous he was going to come in but I don't think that was it. It really is a terrible feeling. I used to masturbate alot and many times immediately post ejaculation (so I'd go one or two more round past my first ), however after reading that it was harmful I stopped and haven't in two or three weeks period. It's embarring, but I don't know what to do. I'm 17 and should be working into my prime. Can anyone help?