I read this last night before bed and was too tired to write out a reply, but seeing as nobody has written anything yet, I guess I could try to relate what I do in these kinds of situations.
I've been writing for over a decade now, and I can tell you - this sort of thing doesn't get any easier. We are our own biggest critics, especially when it comes to art and writing. What I do when I get in a "funk" (as you put it) is either work on something else (another project) for a little while or keep writing.
If you're stuck on a chapter, for example, and you've written it out but you just don't like it, then move on to the next chapter. I find that doing that gives me a fresh perspective sometimes. Going back and changing things isn't too difficult after all, especially if you haven't posted yet.
Another piece of advice I can give is to try to find someone who might be willing to beta your story, at least partially if not all of it. Having someone who is outside the project look at it can give you valuable insight and can give you new perspectives.
Last but not least, I suggest that - after some consideration - you think about posting anyway. People here are really supportive. Even if there's room for improvement, getting suggestions for getting better is preferable to being stuck and not writing at all, right?
I hope this helped at least a little ^_^