If you've never experienced infatuation for anyone by now, then maybe you'll never.
As for those relationships you envy, sometimes they aren't interested in one another and simply focus on other things but they don't kill the relationship, then later they are good to go.
When it comes to sex long term, I can only speak from a man's perspective, and it's along the lines of putting out as opposed to taking in. By that I mean, make it about her pleasure first. Being you, I doubt that's ever the case as it's easy to see you don't care all that much if they enjoyed it as much as you did, then you might find sex with them is a lesser experience than masturbation.
I don't claim to have a solution for your case, you need a woman who can put up with your bullshit and is able to keep you in check and you must set a goal to make their experience with you is better and better, even when you're not as good as last time, you must persist and figure it out. Also there is no point in even trying to do this if you're not into her. To say the least, for you an open relationship is a good start.