I have known writers who work on one manuscript for years. They write, edit, revise, edit, revise, edit...a neverending cycle.
Sadly, most first manuscripts never get published. Writing is a learning process...a continual learning process. In order to improve the craft, a writer must write new material--each and every day.
Don't get caught up in perfection. Nothing is ever perfect. All you can do is your very best. Make sure you have no plot holes. Rewrite passive sentences to make them active. Use deep POV when possible. Check for spelling errors. Once you have done all this, then it is time to submit...and write the next book.
Good luck, and thanks for stopping by!