Writing the first draft can be the best part of writing. It's when you're exploring the ideas and letting them run wild onto the page. At least, that's what should be happening. But occasionally you may start to worry as you're writing. What if a certain part doesn't work? What if the whole story doesn't work? If you let these thoughts creep into your mind before you even finish your first draft, it can really mess you up.
The whole purpose of the first draft is to just get it all out. Every possible thought that you could have for this story should be on the page. It might not all work, but it's too early to know that yet. Even if you've outlined the entire plot before you started writing, you're still not going to know if every single thing is going to work. You'll need to have some retrospect--get the whole story down and then look back to see how every part fits together. The second draft should be for going back, cutting things out, adding things in--really seeing how the story works as a whole.
This is definitely a case of "do as I say, not as I do." I was thinking about this subject because I couldn't quite figure out why my first draft was taking me so long. Yes, I have a sporadic work schedule and a life to fit in, but those can't be the only reasons. Sometimes when I sit down to write, it's hard to get the words down. I try to tell myself that it's ok for it to suck, just get it out, but I still can't do it. I realized it's because I've been over thinking the whole thing.
I think I have some sort of adapting-a-short-story-into-a-novel-itis, where this is the first draft of the novel, but not of the story itself. I'm trying too hard to fix things as I write them, and not after, like I should. I'm treating the novel too much like a second draft. So I'm constantly doubting myself along the way--trying to write the middle of the book and figure out the ending at the same time, thinking about all of the things I need to change in the first few chapters before I even finish. It's just too much.
It's important to take things one step at a time. If you think of things that need to be fixed, make a note of it then set it aside. You don't need to fix anything until you have a complete draft in your hands. Let yourself get caught up in the whirlwind of first draft writing. This part is the most fun, so enjoy it, and don't over think it.