Chrissy, Yup, it is frustrating on both sides. I've realized in trying to deal with it on the HD side that part of the problem is perhaps that the HD also doesn't know exactly what he wants. Sure, it seems obvious to him from the outset (Cemar, for example thinks he knows exactly what will fix his marriage, he is in that if only she would do x stage...Having been there, I know that is not the fix he is looking for, but he won't know it until he gets past it), but as you work closer together you start to realize that it isn't just about sex, it is about fostering a close emotional connection. Until you've worked through the sexual frequency, you think lack of sex is the root of the problem: it isn't. Then when you get the frequency down, you discover that it isn't just the frequency it has something to do with the quality of the encounter as well. At first you think it is just the mechanics, but you soon discover that no, it is something else. It can seem like it is a lack of willingness to the HD, but alas, that isn't the whole enchilada either. This is about where I am in the journey right now. Kind of at a disillusionment stage where it is finally sinking in that she isn't just like me, that she doesn't express her love in quite the same way. What bugs me is I can see how hard she is trying (God I love her for that), but it seems to me it shouldn't be that hard. Well, I think I am seeing that yes, it is that hard because she is trying to please me in a way that is not natural to her. It is like two people who don't have a common language trying to discuss the finer points of rocket science or something. What is sinking in through my thick skull is that my way isn't necessarily the 'right' way, and neither is hers. I am also getting the sense the disconnect has more to do with my personality than it does with her, and it kills me to see the pain I've put her through getting to this point.
So guess what, as an HD I am just as clueless as you as to how to establish that emotional connection (although I think I have figured out that it isn't the sex, rather it is communication). Unfortunately, this lack of a clear path to intimate communion that makes it look to the LD like the whole thing is a moving target, that no matter what they do it isn't good enough. Like you said, that can build resentment on both sides of the fence, and is certainly very frustrating for everyone involved.
I can honestly say that my marriage is a lot stronger than it was a year ago when I started this journey. I am also the first to say that it feels like it still has a way to go, and that I haven't a clue as to what I am doing or where it will wind up.
So try not to feel defeated, but do try to open up communications. I am quite sure it is the only way out of this quagmire. Try to both really listen to him to gain insight into what he is feeling rather than what he is saying, and also try to communicate to him what you are feeling. So yeah, this relationship stuff has gotta be the hardest thing I've ever attempted in my life, and I've got a fairly impressive list of accomplishments if I may say so myself. First step I think is to unconditionally commit yourself to the marriage (and I know some folks here will flame me for that, but as long as you have one foot out the door you can't face your demons and without facing them you can't slay them).