Every writer whose art remains alive is still learning about the craft. Each blank page presents a new challenge, and the challenge defeats the writer often enough to keep the outcome in perpetual doubt. Most likely none of us will ever reach the moment of satori in which every word we inscribe is the perfect one for that moment. To write is to ride parallel lines that may meet on the horizon, but never right in front of you. Perfection is a goal not meant to be realized.
This is why writers often say that a work is never finished, it's only published. Why? Because every day an active writer learns something new about writing, and that something would have caused a change to this word or that word in the manuscript.
Your manuscript is a document that expresses where you are as a writer while writing it. But if you're growing as a writer, the manuscript you finished two weeks ago no longer completely represents where you are as a writer now. This makes some writers compulsive rewriters, revising again and again until the work is published. You probably need a second draft to incorporate what you’ve learned in the process of writing the first draft, and you need a third draft to tie together lessons learned while writing the second draft. No amount of improved technique, however, can replace the white-hot moment of creation, so if a major revision is needed, you’ll need to find that moment again and write from the center of it.
Remember, the things you're struggling with today will be second nature to you tomorrow. Tomorrow, however, you'll be dealing with a whole new level of issues.