Its quite a common "girl thing" . Women aren't that hard to figure. They only want one of two things, speaking in general terms. More, and Something Else. They find something they like and keep wanting more of it, right up to the moment they want something else, which they find and then want more of that. I have no idea why it is so, but as a working hypothesis it works well. <br />
All we guys have to do is figure out exactly when this change is going to come about and anticipate the next need. So don't feel bad. There are lots out there like you. They just never reach a steady state where "Enough is Enough" and they can relax and enjoy it like most guys.
He is as good as he will be. Can you picture a future with him? if yes, work on for filling that future. If not, stop leading him on. It is not an easy decision one way or the other, but give it some thought, and then follow through. You recognize your short comings, which is great. So you are better then you think :)