Start with reading DR. It's the updated version of DB. Read about the Stop Sign technique in there, it will help you when you swing to the negative thoughts and think about your W and OM.
DB basics:
1. 180s 2. Detach - what she says and does doesn't affect you 3. GAL
I noticed you mentioned she "still" blames you. I think that is normal. My W still blames me and we are almost 2 years into this. Just be leader in this area. Learn to listen and validate instead of argue here.
This OM is a loser clearly. What do you do? I'd suggest being a great earner, dressing well, being mature and well spoken. You know, everything that a 27 year old basement dweller isn't. The OM in my sitch lived at mommy and daddy's and worked part time. I just worked harder and started earning more money and that helped me along. How do you know that she is going to meet OM's family?
I think you made a mistake moving out, financially and relationship wise. I don't know what the rules are here but I wouldn't want to be living in a rental while my W was out getting cozy with some 27 year old basement dweller and his family. I'd be willing to bet something happened on that trip. I imagine she has been bringing the OM along while she prepared to break the news to you - that's how this works usually.
H 34 W 29 BD 3/12/18 Divorce Busted Spring 19
It is not things that bother us, but the stories we tell ourselves about things.