Aaron Doughty explains why you can't stop thinking about someone: you're trying to control and possess them because attachment feels like safety. That need for control usually started in childhood when you didn't feel secure. Letting go isn't an action, it's accepting you never owned them in the first place.
One of the things people don't understand about attachment is that they think that the letting go is an action, when in fact letting go of a person, of an idea, of a thing is really more about an acceptance. The acceptance is that at the deepest core, you own none of it. So when you try to make something a part of you that is not you, you begin to create resistance in reality because you're trying to claim something that isn't yours to claim. It also many times is connected to our desire to control reality. I mean, think about it. The people that most often find themselves wanting to let go are people that most likely in childhood may have not felt safe. So what happens is if you did not feel safe in childhood, you may develop a mechanism inside of your consciousness where you believe that in order to feel safe, you must attach. You must attach to people, you must attach to things, you must attach to outcome. And the belief is, if I attach to it and if I desire it and if I try to control it, then I can allow myself to feel safe.
