Sara and the others have given you really good insights, I think. I also found the book "After the Affair" helpful, although I probably ought to be reading something called "During the Affair", as my H's affair continues, albeit perhaps not with the level of passion that destroyed my marriage.
What has been most helpful to me, in ridding myself (though not completely) of obsessive thoughts about the OW, has been a change of job. When my H's affair began, I was working in the public library in the lowest possible position, shelving books. There, in all that silence, in my relatively isolated position, I found my mind spinning round and round my situation, dwelling on every slight, every pain--devising smart remarks and diabolical revenge. Fortunately, when I was promoted to librarian, I had a good deal more to fill my mind. I had people to talk to and joke with, and patrons seeking help for a wide variety of problems. It gave me a feeling of self-respect to help the homeless people who came to me. I felt that I could really change lives for the better, and I worked hard at it.
I can't say that I've got the OW out of my mind, but days go by without a thought of her. I wish I could say that she was beneath my consideration, but, in time, it could happen.
A hobby is only good if it can really occupy your mind and give you back your self-esteem. Don't ever discount the power of finding yourself useful in some capacity.
