Getting finished with the first draft is a hurdle, and it's a good one, but it's just the first one. Real writing is a whole lot of re-writing. You'll end up changing lots of stuff about your first draft...maybe even most of it.
In other words, you're going to have to write everything twice -- more or less, anyway. You'll have to tweak and perfect, change and edit. You'll have to go over everything and possibly alter all of it before you come up with a finished product that you can use. Writing is rewriting...and sometimes, it's awful.
You should absolutely feel good about finishing a first draft, but you should also remind yourself that now it's time for the real work to begin. You've got to go over every line, every word, every letter. You've got to change this word, take out that one, get rid of that paragraph and write a whole new scene. You've got to go back over it and perfect everything.
You may have to rewrite some parts of the book more than once. You'll get frustrated, and you'll get angry, but you'll get it right if you keep rewriting until you know that it's exactly how it should be. The longer you spend on a book getting it right, the better you're going to feel once you have.