(((bluebird25))) my WH began cheating at his HS reunion. It lasted for a few weeks before I discovered the emails. He had supposedly ended the PA and told her they could only be friends. Of course, he also told her he found her SO attractive and it felt like a HS crush he couldn't act on
Then there was the upcoming meeting where he would be picking her up at her hotel.
All of this after he "ended it." Even after I found out and he sent a NC email, he spent the next few months fantasizing about her and blame shifting. I caused the affair because he didn't get to spend enough time with friends and away from me and our kids. I didn't appreciate him enough. I questioned myself and took all of the blame because if I was the cause, that is something I could fix. Guess what it got me? 4 months of TT, and false R.
Do not trust you WH, he has done nothing to earn it. Showing you the phone and giving PW is only part of the equation. Here is what I suggest:
Both of you get into IC immediately. When you say you haven't been nice since your daughter was born, what does that mean? I ask because I struggled with post partum depression and that is how it presented itself. Loved my kids, but felt really overwhelmed and resented my husband a lot. IC can help tease that out.
Read Not Just Friends. My WH sent me summaries of his thoughts as he went through each chapter. It helped him understand what was so wrong with his behavior and lies. We were in MC for 4 months where he continued to lie and omit things. It was a waste of time and money.
Get a timeline from him. Decide how much detail you need beforehand.
Now is the time to figure out what you need from him and why you were settling for less than that. If you "weren't being nice" after your daughter was born, I would be willing to bet it was because your needs weren't being met. Was he helping? If he was being incredible, then see above re PPD. It is no excuse for cheating, but it is something worth looking into. You will feel a lot better with treatment.
If he complies with all of this, then start MC. You can start earlier, but my guess is he is still foggy with regard to the cheating. I would highly recommend reading Not Just Friends before starting MC.