I tend do this also. I'm a bi-polar who enjoys psychology with a fiance who's ADD, OCD, and suffers from impulse control problems (and is a counselor himself)! I can stay in my head all the live long day trying to figure out what's impossible to figure out if I let myself. Negatives I can do easily, but mostly it's just projection and not reality in my case. Or painting past experiences with relationships onto this one. I'm trying to use mindfulness techniques to stop the thought patterns. This will help me experience the relationship for what it is day to day. Easier said then done. When I have the obsessive thought topic come up, I acknowledge it and envision it floating away. "Yep, there's that thought I don't want to think about. So off it goes". I might find myself repeating this over and over. But the point is retraining my thinking. It has helped with other thoughts that became obsessive in the past. Otherwise, I try and keep my focus on whatever it is I'm doing at the time being.

If the relationship is unhealthy. Being in the present while spending time together in this way (casting out obsessive thoughts when we recognize them) will be easier to see the quality in the time we actually spend with our partners. Or lack of. ;-)