I'm sorry you're hurting.
My partner is monogamous and trying to come to terms with having a poly girlfriend. Unfortunately for him I didn't make it clear how important that part of myself was when we first started dating (because I didn't actually realise how vital it was and that I couldn't just ignore it) which has complicated things for him hugely.
I'm no expert on mono/poly relationships at all. I'm still trying to learn how not to hurt my man by doing things which I feel are normal but hurt him deeply. I think that might have been what happened with your lady-friend. She didn't realise that talking to you about her relationship troubles with another lover would upset or hurt you.
Right now, my partner and I are working on boundaries (more coping strategies really) to help him feel more reassured and secure in our relationship when we do get to the stage of me getting another partner. I'm not in love with anyone else right now so it gives us time. There are people I do love and could easily form deeper relationships but I'm putting building this one with him first. I know it's most likely harder for you because she wasn't single when you met.
Do you know if you two have the same goals? House, family/kids or lack thereof? I think when looking at the "future" of a relationship, even if it's mono/poly, it's important to evaluate the relationship on it's own merits. That said, mono/poly relationships are hard (I've only just really started mine, been with him 1.5 years long distance) so it's important to explore if you feel you can ever be comfortable or happy in one. I wish you all the best.