Wow, the waywardness mind never ceases to amaze me. Do your fair share of finances, but no more. Validate his feelings, but set your boundary regarding not talking about OW, next time he mentions her just say that you will not discuss OW, and end the conversation.
Also, don't generally have long conversations with him, cut conversations short, listen to what he has to say, but when he expects you to provide input or responses, try and end the conversation.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized