A little Context:
I'm currently with a wonderful woman and have been for 5 years. On a whole our relationship is sound. I had strong feelings for a different girl when I was younger but a relationship never materialized. There was always a strong feeling of 'What would it have been like' and there's still some of those lingering feelings today.

The Dreams:
I've been having several dreams a month about this old childhood sweetheart. The dreams vary from just appearances from the old flame to full on centralized dreams around the 'what would it have been like' subject. A lot of the time I can't remember what the dreams were about but i just think about her afterwards. This puts my mind in a strange place for days and it affects my relationship with my girlfriend (without her knowing too much).

Has anyone had anything similar to this? What on earth do I do?

I'm still in contact with the girl from the past because we've known each other for 18+ years and I don't like losing contact with people from my past that I enjoyed time with.