Oh. Not easy. Like Hankster says, it helps if they key your car. Or constantly tell you you're stupid for having a different opinion to theirs. Or expect you to be happy when they 'give' you what they want. Or get upset when you do things for you. Or only throw a bone in your direction when they want something. Or... etc...
It's f****** hard, but every time you want to obsess and cling to this person/habit/whatever, remind yourself of all the sh^tty things they've done to you.
This morning I was struggling through a moment of regretting some of the decisions I've made lately, but then it struck me that the idea of my unhealthy relationship is nothing like the reality of it. It's the idea I don't want to lose, but my idea will never become my reality if I don't let go of this relationship.