Joannie, you are on a long hard road.The obsessive natured pain,is absolutely part of it, which in my case would come and go in devastating, frequent "gear change" cycles.When it gets better is goign to vary from person to person; we differ as to persons,the betrayal, the actions of our partners, the circumstances after-yours,having to see the other woman, is tough.
For a time,the pain is unavoidable.We are processing immense injury.But after a time, the pain (perhaps)always there at a deep level, is a major part of the injury when it changes to obsessive-when you can't move on beyond it. Me. it took six months to get out of the disfunctional zone; perhaps eight until I got some of my mental freedom back.
The pain focus on the OW will make its own trending. Every time I think you can beat it back will help beat it back long term: small victories matter more than like scaled defeats. Try to take the anger at OW and put it as fast as you can,in to some specific project.Try the common mental "trick" of seeing it as a tv channel-and mentally change the channel-this is like yoga. I do feel a keeping a journal can be a real help in controlling the emotional fall. You open the cover,you express your feelings-- you close the cover.Just the "objective expression" can help break the run away pain.
My case was different,I hated my wife's lover so much,my "best" friend, I was able to (mentally) "kill" him off very early on.There are good people here and do stay tuned for their better suggestions. You know this but may I say, the OW is human, all too human and is not worthy of your anger-hate because YOU, the innocent, pay the price of it, not her. Joannie, it will get better.