ISWG: First-Timer

This is my first post for the Insecure Writer's Support Group, created by Alex J. Cavanagh. We post every Wednesday of the first month about any doubts or concerns we may have without appearing foolish or weak for the purpose of supporting and encouraging one another. Check out the group here. A big thanks to Alex and the co-hosts this month: LG Keltner, Donna Hole, Lisa Buie-Collard, and SL Hennessy.

A big insecurity for me is being vulnerable with my writing. I've finished a manuscript I'm really proud of, but I'm worried it doesn't have a place in the marketplace. I'm not sure if it's too niche or too played out. All I know is, I really just love it to pieces.
I keep coming back to it, even though I should be working on something new. I wonder if I should go back and tweak it, because if I don't, I fear that it means I've given up on it. When do we know when it's best to leave it alone? Will it do more harm than good to revise it again?
How do you know when your manuscript is 'finished'?