Can't let go of the past, should i seek therapy?

I don't know if it's cabin fever or the lack of friendships but I find myself thinking of the past too often. The what if? And shoulda coulda woulda. Usually when I speak on it, people say it's normal and to move on. I do and for a while I do well, but then it creeps up like roaches in a dumpster in the back of a buffet restaurant. It doesn't just cross my mind it will take over my mood and sometimes ruin half my day. I thought it was something i worked through, but it comes back. Similar to a haunting nightmare only more petty than crucial. Is this normal? Do others go through this?

Ive been doing this myself when it comes to relationship's. After almost 4 years Im still trying to figure out if I made the right decision not moving in with an ex, if I should have stayed with a great guy I was attracted to, or if the man Im with now is the right choice. I wild suggest therapy because that is what Im seeking now.