Flankert, I realize my answer comes in waaay too late. But since you basically described my life situation in your post, I thought I'd comment. Don't feel guilty about anything. It's not that you don't love her enough, it's not that there is someone better out there for you. None of this crap. The truth is we are cursed! We are doomed to always be sexually unsatisfied. That's how nature gets us reproducing.
You've got a couple of options: either you become single and try to sleep with as many women as possible, perhaps become an asshole, cheater, etc. You may have a fun life for a while, especially if you've got no moral values and no guilt. (You may get a bunch of sexually transmitted diseases on the way, and keep transmitting the beasts.)
Or, you've got to learn to live with this painful situation with no hope to ever be satisfied.
I've been struggling with this situation for years. There is no fix for this. All solutions are short term only. We are cursed. Sorry to break it to you buddy. It' just something you gotta learn to live with, accept, and then perhaps when you are resigned to it, you may have an ok relationship. Perhaps.