Is it really obsession? What you describe here sounds to me like a common reaction in healing from the grieving process. Is your mental rehearsing of your therapist's insights due to an inner struggle as you process your feelings into the acceptance phase?

Illustrated below is the grieving process defined by Elizabeth Kubler-Ross. In the depression stage, self-blame may occur. The process isn't linear as people cycle through these stages. It's normal to move to other stages then find yourself back in the first DENIAL or other phases. The point is, it's a process. It doesn't happen overnight.