FFHubby, welcome to the board and the same time, sorry you are here. However, you will get a lot of great advice on this board.
I want to start with a question, because a lot of newbies make this mistake. You keep talking about your sitch being "unique". Why? Is it so that you can say that you shouldn't stick to the script of detaching, GAL, 180ing and LRT? Because your sitch is more similar than dissimilar.
Here is the thing you need to wake up to: your W has a OM. Oh it may not be another lover, but a meddling father, trying to convince his daughter to leave her husband IS an OM. No matter what his reasons. Now obviously if your marriage was stronger, that influence may not have had an effect. You have no way of knowing that for sure.
As far as her letter. Yep, typical WAW script. Take your conversations about spending and turn it into "verbally abusive". You're selfish because you wanted to save money. You were emotionally abusive because you were depressed and withdrawn. And you didn't put her needs first, which isn't even always the right thing to do anyway!
So you see, your WAW sitch is TEXTBOOK. And therefore needs to be dealt with in a textbook way. You need to heed everything Cadet sent you. Detach. I also believe LRT is the right approach in your sitch. DO NOT INITIATE CONTACT. The longer you go without initiating contact the more curious she will get and she will eventually reach out. I would make her do ALL of the work for the D. Don't do anything for her. If she says "I need a copy of the marriage license for the D." Your answer? "Sorry, I am too busy to help you with that." No matter what the request, you are too busy, don't have time. Etc. She will inquire, why are you so busy? Don't answer. Let her mind wonder.
Most importantly, GAL. SO that you ARE busy. So that you ARE occupied. You mentioned being a wreck. That tells me your GAL could use some serious work. Likely you will not be successful, whether she comes back or not, without GAL. GAL is the key to detaching. It is nearly to impossible to move on otherwise.
FFHubby, you got this. You are a strong, secure, confident dude. Your life is going to be just fine with or without her.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018