I love counselling / therapy. I firmly believe that there is no person on earth who can't benefit from regular, or at least occasional, therapy sessions.
There are definitely some issues here completely unrelated to poly or marriage or anything relationship-related.
You need to focus on yourself and find out what you need to make yourself happy. I'll get you started: it has nothing to do with any other person on the planet, not your mom or your fiance or the cute boy at the post office.
I think one thing that can help boost your self esteem is volunteering. I'm thinking something like Big Sisters, or visiting with old people in nursing homes. There are lots of people out there with the same kinds of issues you have, and helping them is a great way to help yourself. Plus, it gets you out of your mind for a while, lets you focus on something in-the-moment, and you're sure to get some gratitude that will make it even better.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker