No, there is no medication that will speed up your orgasm.
My advice? Relax and enjoy what's happening. Try out this novel concept: Sexual activity is NOT a race. It's about enjoying what's happening at the time, not about having an orgasm at the end. Some men can't orgasm at all, because of medical problems or whatever, but they still very much enjoy sex.
You say that you can orgasm, it just takes a while. Here are some ideas:
1. Do you orgasm sooner when you masturbate? If so, how about masturbating with your girlfriend watching so she can see how you do it and maybe get some ideas of what works well?
2. You could talk to her about what feels good as it's happening. Communication is a good thing!
3. Variety is the spice of life. If one activity isn't making you both happy, try a different activity.
Again, sexual activity shouldn't be a race. Think of it more like a river float trip, where the point is to enjoy the journey, not zip to the end.
Here are some other parts of this site which may be helpful:
Orgasm and Sexual Response: A Users Guide
The Men's Room FAQ has a couple of "no orgasm" threads which may offer some insights.