Do you have children? My partner is also going through a divorce. It really sucks. His story is somewhat similar to yours. He's always known he was poly,but married a mono wife much too quickly when he was young. They had a child, experimented with poly (ie they met me and we started an open triad) and all the numerous problems in their lives kept piling up despite my presence. Surprise, surprise.
It's been incredibly tough. Basically I'm also going through this divorce as well. I did all the ex- spouse support things like helping hiw experience pay for her new place. It seemed only fair. Now I'm going through the emotional devastation of a custody hearing, without the ability to do a damn thing about it. I've become a mother to this child and now I'm facing the traumatic possibility we could lose her.
Divorce is tough. Stupid tough. By the weirdest thing is I'm actually happier now than I was with her. And my partner is gaining weight, starting to look healthy again,after years of anxiety and pressure on him. So that's to say... It is a hard thing to go through, but life really does get better.
The reasons you find yourself leaving your partner won't likely change. And you'll begin to discover the happiness and freedom from stress you hoped forgot years now.
Best wishes, best luck, best thoughts.