Yeah, well. Fretting is part of the fun? No, seriously. You'll be fine. Just keep putting words down. Write all the parts you know and then see what you have.

But keep meditating on what your MC wants. I know that can be a very hard question to answer when the characters come to you without a complete story. I find it easier in some ways to deal with stories that come plot first and characters second, but I am rarely that lucky.