It's a crappy feeling when the ex seems to have moved on so easily and effortlessly. I've been there, and I know others here have too. I've been in the other position too, where I'm sure I looked like I had moved on way too easily.
From my own experiences, I can tell you this:
- It is an act. I know I was acting like I didn't care, because I didn't want her thinking I was some hurt little puppy that couldn't get over her. The truth is, once I was alone I'd go back to being hurt and on the verge of tears.
- I actually was pretty happy it was over, in a weird way. There was a lot of pressure and stress leading up to the breakup, and after it was over the pressure was released, and I felt a lot better.
If the problems were mounting for a long time like you said, then I'd be leaning more towards #2. She may be feeling like a huge weight has finally been lifted off her shoulders, and is feeling pretty good about it. That doesn't necessarily mean she feels good about hurting you, but feels good that the stress and pressure is finally gone.