To chime in on what everyone else is already saying: what you're feeling is perfectly natural!
I still think about the boyfriend I had before I met my husband, even though we broke up nearly 7 years ago. I wonder how he is even though I know that if we had stayed together, it would have been disastrous.
It's normal to wonder "what if?" But as much as I wonder, its great to sunggle up to my husband at night.