I used to have that same problem, but it mitigated over the years by practice. Now I'm feeling relatively confident in writing those middle parts. You just have to make sure that your every paragraph feels like they're tightly connected to the main story, instead of having some just loosely hanging on to the general story structure.
That said, I speak from the viewpoint of an outliner(not a full-blown one, I tend to discovery write my characters and those "middle parts" you mentioned). It might be that your writing method is a bit different, but the same principle from the first sentence applies - just write. You'll get better. If you feel up to it, I would recommend reading about writing(but if that puts you down, just write), however, even with all that knowledge that you could gather, you'd still have to learn to put it into practice. And I can't stress enough how much a good night's sleep will help you to write better on the next day...
sighs Sorry. I got started on a rant. Just keep in mind that writing is a holistic art - a lot of things influence your performance. XD