Robs,
I do not want to be too judgemental nor to be critical over other people decisions,at the end of the day we are all individuals and make our own decisions.
If you ask me my opinion ,I think ,from the looking at cards ,it sounds selfish as it looks ,she is just wishing to be herself.I agree ,its very selfish,but you need to understand there are a lot of young women out there still looking for identity;who they,what really want..etc.as for her,it might not be ‘other guy’as you think ,he might happened to be there for her,someone soon to be fallen into waste category in her terms.
Once she has found ‘real’herself,she may come to terms of forgiveness or you see some kind of maturity from her if you had child together,but I would not fall into long term relationship with her or let her ruin your potential relationship to other women.
I would just suggest you ,though its hard to swallow it all,be brave and take your control of your own destiny instead of your depressing all your life for nothing.