Selecting People For The Relationship Work

I just read through the May materials on relationships. Do we need to select people who are currently in our life to focus on? I’m thinking of using someone that I recently ended a relationship with. I don’t want to resume my relationship with them, but I would like to work through some resentment that I still feel. Is this the right idea? Or do you suggest focusing on people who I will continue to have regular contact with?