When you write a melody, what matters generally made you decide that which one try is the better than others? I often lose criticizing after listening to tries too much.

I'm not sure I really understand what you're asking. Are you asking about rewrites? How to decide you're settled with what you wrote?

I think you need to start, then continue. Don't keep stopping and rewriting constantly try to finish, then do your edits. Too often what many of us do is not finish stuff because we go back, make an edit, make another edit etc. and may start to lose either the vision or the feeling and the seed gets lost. It's important to finish even if you don't love everything you wrote. Learning how to draw up an ending and how to resolve various elements is a lesson in itself.

Is that what you were asking about?