Yes, very. I have these thoughts too. I actually have them about 2 of my exes. But I am happy with my current gf.
It's perfectly normal. It's certainly easier to wonder what would have been with someone you know than to look to the future and think about someone you haven't met yet.
Absolutely. The image of your former partner is well-established and your mind summons it more frequently than any of the other people.
