I'm sorry you haven't had replies as of yet. This can be a very serious issue to your life if you don't figure it out. I hate to raise the alarm bells, but Anxiety about Erectile Dysfunction (Going soft, not being able to get it up) is a popular cause for erectile dysfunction. I was in the same situation at 18.

For me, it was simply this; I was nervous initially about making her orgasm, and anxiety in any form is a real boner-killer. Then, after a few times, I became anxious about getting it up. It was a cycle of anxiety that fed on anxiety. If I can offer any kind of advice it would be this: Just relax. She's giving you plenty of chances, so she obviously is patient and she really likes you, and she isn't going anywhere. Focus on the good feelings, focus on the things about her that make you aroused, and focus on the things you'd wish she'd do (Fantasy about your partner is a powerful thing), to take your mind off your present worries.

The biggest thing for me was to attack the problem at it's core. Either manually, or orally I would bring my girlfriend to orgasm, and then I was comfortable enough after to focus on myself, relaxing, and just enjoying sex.