Not that I’m an expert or anything, but it sounds like a psychological thing to me. I had a somewhat similar problem when I first started having sex: I couldn’t finish. It was, literally, years before I was able to have orgasms consistently with a partner. What “fixed” it was getting comfortable with the whole sex thing and learning to let go and enjoy things. I think I had too much anxiety about diseases and pregnancy when I was just starting out. When I had my first serious long-term girlfriend, the problem largely resolved itself.
Things that might help you:
Do you have trust/anxiety issues about sex? You should probably explore your feelings about sex. If you do have hangups, are they related to your girlfriend specifically, or are they about sex in general?
Are there positions that really get you going? Sometimes a position doesn’t turn you on enough. I usually need pretty strong stimulation to finish, even when I’m close, so a position like missionary, with her legs raised so that I’m very deep in, or rear-entry helps stimulate that sensitive ridge on the underside. Plus there’s the psychological factor related to those positions. Find out what works for you.
Are there sex acts that turn you on? I find it easier to finish if I’ve had some oral attention to start with. I also get off on getting a girl off. If she’s had some good orgasms and is really excited, I get more excited too. Reducing her to quivering jelly might give you a nice ego boost and help you stay with it.
How long are we talking about here? When I was having trouble finishing, I would sometimes go for a long, long time. Sometimes I was hard the whole time, on other occasions, Lil’ Sleel wouldn’t cooperate and stay in the game. If we’re talking an hour or more, it might be just that you’re tired, or overstimulated, or picking up negative feedback from your partner: “You’re still not done?!”
Are you a Top or a Bottom? These terms apply to more than bondage. Some people really need to feel in control and only get off when they’re on top or in a position of psychological power. Others only like it when they’re subordinate. Which are you? Or are you somewhere in between?
Definitely not meant to be insulting, snarky, or implying anything, but are you sure of your orientation? When I was in my early exploring phase, I found that guys don’t do much for me. I tried some stuff, but it didn’t work too well. Even just the smell of a woman gets a reaction. I can admire a good-looking guy, I might even be able to picture doing things with him, but it doesn’t get me excited in anywhere near the same way that a woman would.
Worrying about things too much is counter-productive. Figure out what you like and don’t like. Get comfortable with your responses and get comfortable with your partner’s. Relax, have fun, and most problems will probably work themselves out.