Sounds like you really can't get this out of your head. You sound like me awhile back. Talked to my T, she advised me to go ahead and allow myself to think about it all I wanted. Once I had that kind of permission mindset, I still had some obsession but realized I thought about it less and less over time. Yeah, it did bug me that these were intrusive thoughts, but once I quit fighting them when I found them in my head, I didn't beat myself up emotionally, I just noticed it and continued on with my day (although it really bugged me). It took me a long time to stop obsessing, but it's possible.

You could also try and manipulate the obsession, every time you think about her with him and catch yourself, replace him with you. Focus on her, keep that focus on her and good things.

Hope this helps and good luck to all three of you.