I read everything you wrote and honestly... there are a lot of issues between the two of you. Have you two had any space? Are you living together? Did the cheating happen two months ago or have I gotten that wrong?
You should definitely take a look at the life stories and blogs section. As for me personally - I didn't think much about open relationship models. I don't feel like i was 'born' poly. That being said, I've never had good experiences with monogamy. Poly, for me, is a life choice made on my opinions and viewpoints about monogamy and how I haven't seen monogamy work in my life.
My first girlfriend was ten years my senior and was cheating on her fiancé with me. My boyfriend after that cheated on me twice. My boyfriend after that - we were mono for 5 years; but I had always been tempted by other women. Eventually we became mono/poly and I dated a woman at the same time as him for six months, before breaking up with him and being with her for three years. She had a wandering eye and I ended up meeting my current girlfriend towards the end of that relationship; who i am poly with. We are only poly because my GF insisted on it when I met her and I liked the idea. I just accepted it and we got into a relationship. The choice was as simple as that. The hard work comes after making the choice.
Poly isn't without its troubles, but I struggle to think of being mono again. Yes, there is a concern about cheating - though cheating can still happen in poly. But, overall, I like the freedom of being able to act on it if I find someone attractive.
One thing I have learnt is that adding poly to a cracked foundation can be a recipe for disaster. If this other guy wasn't on the scene, I'd be suggesting that you two do some self and relationship work before opening up. However, she still has feelings for him... So what's going to happen there?
If i was being completely, completely honest... I'd be wanting to take a step back. In all honesty, I'd take some space and open up the relationship, whilst continuing to both work on yourselves. Everything sounds very heavy in there right now. What can you do to lighten the current situation?
me: open poly (31, female)
GF: (41, female) my long-distance, long-term partner
Earth: (35, female) newly dating
Hubby: (38, male) GF's husband
Garcon: (28, male) GF's boyfriend/submissive
“Peace comes from within. Do not seek it without." ~ Buddha