I would really suggest you read How to improve your marriage without talking about it. It really explains how men and women see things different and why you thought everything was fine and were blissfully happy with a new hobby.
I once told my ex I only went out with friends because he never wanted to go and he said yeah, you left me only so I could do my hobby. Err not the way I was seeing it.
She is not in a place to work on your R. If you push her on this OM, checking up on her, etc you're only going to push her further into the R.
I'm glad you're seeing a therapist. Talk to them not your MIL, not the rest of your family. You don't want to put them in a place of taking sides (even though it feels slightly comforting to think they sympathize with you) or making W feel like anyone's ganging up against her. Again this will only push her further into her decisions..
Someone will post the 37 rules or do a search they are a good plan to start with.
