Wow, you people have absolutely no idea how homosexuality works.
OP, you're gay. There is nothing that you can do to change that; you were born a certain way, and that orientation is completely and utterly set-in-stone. You may love your girlfriend, but in the end, it simply cannot work out. You've lied to yourself for far too long already, and the further you get into this the more it's going to hurt her in the end when you finally do need to tell her (or worse, when she catches you in bed with a lover). If you truly love her, then you want what's best for her, and you know that it can no longer be you. Taking pills to get an erection and just trying to muscle through sex is nothing more than an elaborate lie.
Once again, please don't blame yourself. It's not your fault. In fact, it's nobodies fault. I may not exactly know your position in not feeling sexually attracted to her -- I've had successful and passionate relationships with members of both sexes -- but I do know what it's like to deal with the fear of "coming out." I've sent you a PM in case you would like to talk further.
Oh, and yeah, this was pretty much one of the worst places to ask your question. Styleforum is very, how you say, old fashioned, when it comes to this. But I wish you the best of luck in ending your relationship.