Sucks, but it kinda depends on the problems. If it was something insignificant, you can rest assured she'll move on from him too should things go in the same direction.
And I don't think it's reasonable to generalise. Some people might be like that, but I've found most people are willing to work on problems up to a certain point. Everyone has a point where they have to break things off for self preservation purposes.
Her reasons for breaking up were she hadn't been happy for a while and things had become more of a routine. Which to be fair it had. Working so close together was great at first because it meant we could meet up for lunch, go straight out after work together, go shopping together etc etc. As things progressed she suggested I park at hers (to save money) and walk to and back from work with her. This meant we pretty much saw each other every day the year we were together. I think the longest we went without seeing each other was 5 days and that was because I was in another country but even then we talked all day every day.
I think this caused a lot of codependency on her end, she even told me there was times when she would cry when I would go straight home after work. But It wasn't because I didn't want to see her, I just knew seeing each other every day wasn't good for us, we both needed space and alone time. I tried changing things up, I suggested we did different stuff but it was so hard to when she claimed she was always tired. When we first got together we filled our weekend up with activities together but the routine then became, picking her up from work on a Saturday evening, have something to eat and then get into bed for 'Netflix and chill', she'd then fall a sleep pretty early on. Sunday it would depend on what time she woke up and how she felt. Sometimes it was a day out somewhere, going out for lunch or a trip to the beach but 9 times out of 10 it was spent in bed together or me leaving early afternoon. Which did frustrate me because I wanted to get out and about with her.
With regards to her not being happy then yeah I possibly could do nothing about this, you need to find happiness from within but it still sucks considering days before this new guy came back into the scene she had been talking to me about our plans for Christmas, what we were doing for her birthday next month and even plans for this summer. I was still getting the texts about how much she loved me and how much I meant to her.
I think rather than working on what we had she saw this guy as something new and a quick and easy fix to get her happiness and some excitement back in her life. When really she needed to work on herself because she cant find that happiness being alone. I'm sure the honeymoon period going on right now is great, I really do hope she is happy but I'm sure when the routine kicks in with this new guy she'll be back to square one again.