Most Helpful Girl
I disagree with the guys' answers. Just because she seemingly still cares about her ex doesn't mean she doesn't love you or care about you. It's normal to reminisce about the past and think "what if?" but the grass is always greener on the other side. If you truly love someone, that spot in your heart never really goes away completely, you just accept it and move on to a bigger love that fills its space. Do I think about exes? Sure, and sometimes I even miss them as a person. But I would never want to be with them instead of my boyfriend, because they're in the past and I'm with him.
The real issue here isn't whether or not she still has feelings for her ex, it's how/if she acts upon those feelings and respects your relationship.
When you're with her, is she with you (meaning emotionally in the moment)?
Do you trust her? Do you think she would cheat on you with him?
How long has she been broken up with him before you got together?