I'm so sorry. I totally relate to your story as my ex did the same. "There is no one else, I just don't want to be married anymore." "We can have a trial separation and I promise I won't see anyone else until we know what we are doing."
It was all a bunch of bs designed to allow him to leave the home, leave his responsibilities and obligations behind, and play new happy boyfriend with his own secretary. I found out right around this time of year in 2010. I was crushed. Even though, like you, I had suspected, I honestly just didn't think he would go that far as to have an A. He totally blamed me for the fall of the marriage and I blamed myself. I had no idea we even had a problem, but I bought into the idea that it was all me and had I just been ... better ... I could have kept us together. It was all a crock of shit.
The OW in my case is alot like the one in yours. She is a social media whore who is dumber than my desk. She loves attention and loves drama. She also maintained a site where she would talk about my ex - who was still very much married - in code. It was easy to see who she was talking about, but she had no problems discussing their sex life, the gifts he bought her and the places they went. I thought I would faint the day I found out that they went on this exotic vacation together before I had even filed for D.
What I can tell you now, three years out, is that it gets better. If your wh is like my ex, he is a narcissistic, selfish, weak, coward. If she is anything like the OW slunt in my case, she is nothing but a selfish, phony, needy POS who will show her true colors soon enough. They are toxic. No matter what she shows on her stupid instagram, what topics about their life she discusses, please trust me when I tell you that it's all a game of smoke and mirrors.
I honestly believed in the beginning that the OW must be great. I thought she must have been a younger, better, more fun version of me for him to have left.
You know what I know now? She was the path of least resistence. She "won" a liar and a cheat by default because he knew that once I found out there was no coming back. He knew that he had to step on my neck and smash my self esteem to make sure he could continue to eat cake for as long as I would let him.
You want to know what else I know? The OW doesn't hold a candle to me. This is the third married man she's been with. She lost custody of her own children and is, by all accounts, a selfish, shitty mother. She treats my children like furniture. Imagine that? She worked so hard to get him to become a part of her life, but yet, when his kids are around, she could care less if they are fed, clothed or loved. My kids tell me that she yells at him and tells him what to do on the regular. She's "grumpy and cranky" as they put it. The only reason she doesn't scream at them is because I told him if I find out she so much as looked at them cross-eyed, I would pay her a visit that she won't forget. She's nothing but an uneducated, low class, pile of dog shit who had no qualms about chasing after a man she knew was married and had small children. I don't care what she looks like, what she weighs, or how great she thinks she is in bed. She is nothing.
While you hurt so badly right now and, I won't lie, it can take a long time to get over this kind of betrayal and pain, you will be better in the end. You know why? Because you will grow. You will survive this and thrive and you will learn to face pain and hardships with class and strength. He, on the other hand, will stay stuck in the immature, selfish mindset that got him here in the first place. You, my friend, lifted him up. This OW will tear him down, which is exactly where he belongs.
In the meantime, deal with it as best as you can when it comes to DD. I used to want to puke whenever my kids said her name around me. Now, I could care less. I have perfected the fake smile and the "that's nice" statement whenever they talk about her. If you give DD a sour reaction, she won't spill the beans when they need to be spilled. If you stay as neutral as possible with her, there's a whole lot you're going to learn.
Please trust me on this. I share my story with you because I was in your exact same place. There were lots of times that I never thought I would come out of it. While I will never fully get over it and there are a few bad days here and there still, I know in my heart I am so much better. You will get here too. Just trust every day that he is not a good man and he in no way deserves a woman like you.