Thoughts on thinking (fondly) about an ex while in a relationship?

Background: I’m 22, just graduated college and have been with my girlfriend for over two years. Before her, I was with my ex for 3 years until she broke up with me because she was starting college (at the same school) and wanted a fresh start.

The Story: For a little over a month, I have caught myself thinking about my ex and wondering whether or not I’d be happy with her. Yesterday, I had a dream that we got back together and I was elated to have her back. I was happy with her until the day we broke up and I’ve always wondered how things could have been.

I love my girlfriend and, when I think about it, she and I are a great fit and should be very happy. Nevertheless, I’ve always been unsure in my heart that I want to marry her.

Questions: Have any of you had such feelings towards an ex? What did you do? How did it turn out? Do you have any advice about my situation?

Any input would be appreciated :) Fluther is the only place that I’m comfortable sharing this.