I love my wife and kids, the ex-GF is married with kids, but I think about us being together all the time and can't stop thinking she was who I should have been with.
We knew each other when I was a rising star at my company and the future looked a lot more promising. I'm sure my feelings about her have more to do with what she represents - a time when I was on the verge of major success, which I never achieved because of bad decisions on my part and bad luck.
Given our respective situations it would never be what it was. But, almost everyday I think about how much better my life might have been if she had said yes when I asked her to get married.