Indulging in past

I keep thinking of my past. I relive an old relationship, even though I am happily married. It was someone whom I dated for years and I did not receive the closure I needed when I ended the relationship I am trying to figure out, why I am continuing to think about this person. I feel regret, anger, guilt, sadness.

C: my past
T: why did I have to go through it
A: keep thinking of my past
R: I am not present in the moment ( in my current relationship).

Any advice? Thank you