Alison,
I, too have a story to tell about obsessive thoughts in a relationship. If you would like to talk more, you can email me at mare@mail2peace.com. But what I really want to do is point you to a website that helped me and showed me there are many, many others that go through this. The person who runs the site is also a mindfulness meditator and shares his story how he overcame his obsessive thoughts in his relationship.
Here is the link: http://www.retroactivejealousy.com/about/my-story-2/
I hope this helps you in your healing journey.
Mare