Today is the day I tell her. Now I *was* going to let the divorce decree get delivered to her as her official announcement from me, but my therapist felt that was a bad idea and I agree with her. Someone might find out early and tell her or she'd find out from the local newspaper, I have to get over any anxiety, and while she lowered the boom on me with multiple affairs out of the blue: I AM NOT THAT PERSON.
I've struggled with what I'm going to say. My therapist and I were going to work on this, but we ran out of time, but when she started it was firm, short, and basically: if there's anything to discuss it needs to be with a lawyer, and that she needs to get a lawyer. And that I really don't want to talk about anything else until we are with mediators or a lawyer. That's it. Over and over. Be firm and be emotionless.
Today we are having a party at our house for my neice, her Sweet 16. I really love her. We've become good friends because she started acting in community theater. I really want to have a final nice interaction with my niece before it all goes to hell. People have told me that she will understand but I just want one more 'pure' interaction before she might decide she doesn't like me or whatever.

It's completely for me.
If it goes nuts, I have friends who are ready to help and there's a hotel not 5 minutes away.
Anyway this is what I'm after.
Wife:
I have decided that the marriage isn't salvageable and the romantic relationship has run its course. Your continued unfaithfulness has broken the marriage vows, and your unwillingness to seek help have damaged the relationship beyond repair.
I have retained a lawyer, this is a copy of the divorce decree that you will be officially receiving via certified mail this week. It will need to be signed for by you. You should look into getting a lawyer. I do not wish to discuss any further particulars outside of mediation or law offices. I am going to talk to (24 year old kid at home) about this shortly and I am going to leave for a few hours to be by myself.
Daughter:
First and always, I love you. I always will. I need you to understand none of this is your fault. I have decided that I cannot stay in this marriage because of your mom's continued unfaithfulness with other men, which I did not deserve. Your mom has cheated on me multiple times, and I feel she didn’t work on the things she needed to: to help fix why she did it repeatedly. Some day if you want, and when you're comfortable we can talk more about it, but please understand I have to do this for myself. I promise you, none of this is your fault.