OP, I hear you on all of your points. Through years of therapy, I've arrived at the insight that this is due to the abusive relationship I've had with my mother. She "partnerized" me, and would constantly badmouth any type of initiating behavior by men (e.g. in movies, tv shows) as "typical male pig" behavior.

For this reason, I've never approached a woman, even though my (now rapidly fading) looks gave me plenty of opportunities, to which I gave in sometimes but most of the time I rejected those opportunities regardless of any considerations and just like you I've felt uncomfortable in relationships, often feeling like an impostor, that I'm "playing relationship" more than actually being in a relationship.

Unfortunately, the only advice I can offer from here is to explore how your childhood experiences may have adversely affected your relationship and bonding abilities. Even if you think your childhood was just happy and "normal", it's fiendishly difficult for ourselves to judge those things, and human memory is a wicked thing. It's not enough to just remember things, we also have to learn to give weight to those things, and that can take a lot of work.

Other than that, if you feel guilty for wanting to have sex, I'd say the best possible course of action other than therapy is to say exactly this openly. I mean, don't open with that, obviously. "Hey, I feel guilty for wanting to have sex, you down with having sex with me?" :D But, you know, maybe you could try making a point of mentioning it as soon as those guilty thoughts creep up on you and inhibit further interaction. And don't be shy about it. The worst that can happen is that she'll give you a weird look and never talk to you again. Then again, that's not the type of person you would want to have any kind of interaction with anyway. Decent people will be understanding.