I'm in a relationship but keep thinking about a past fling.
Is it because you have issues in your relationship currently?
I don't know, I just think about him when I'm in my relationship
People in a relationship should not think about others who were a fling, it is more peaceful that way.
Are you telling me I can't think about things from the past?
You can think of the past all you want but leave it where it was. That way you don't have to face any repercussions if acting upon a fling anymore.