Need advice badly?

When i'm trying have with a girl i always have problems getting it up when it comes actually doing it. I have an up until the time i take her panties off and my boxers. I'm not really shy because i've messed around with alot of girls but it usually just ends up in oral or something less than . I've had with one girl before and i had to take something to hlp me get it up. I'm not sure whats wrong, I'm definitely attracted to my girlfriend but it just goes limp when the thought of comes up and it pisses me off cuz i want to have with her. Is there anything i can do to help this problem so i can start having sex without problems?