I've posted about this before, I'll try to be concise!!
After 4 years of IVF, a miscarriage and our beautiful little girl being born, I was diagnosed with PND, our DD was 15 mths before I even went to see a doctor, I believe I had it about 3 months after she was born? 
Anyway, I was an irrational, nasty, unpleasant person who was very difficult to live with, but this was linked to the PND. On Boxing Day he told me he didn't love me and left within 1/2 hour, leaving my parents and me devastated!
I had a feeling there was something going on, so looked at his mobile phone bill online, (it was in my name, he didn't cover his tracks very well)!! I found over 500 text messages, loads of calls to the same number, even some on Xmas day when he disappeared upstairs. So I phoned the number and a woman answered! I worked things out and confronted him and this is what I found out. Within 3 weeks of being diagnosed with the PND, he had found himself a 'friend' at work. Spent most of his time texting/calling her, even when he was looking after DD, this went on for three months, he met her for drinks etc and he states he 'kissed' her. He admitted everything to me, defended her telling me she was lovely girl etc. As soon as he left on Boxing Day he called her, and carried on calling her days later all listed on his mobile bill!
His parents found out and he convinced them he hadn't done anything, they were just 'friends' and weren't seeing each other, and it was all my fault our marriage had fallen apart. He painted an awful picture of me to everyone who would listen and I convinced myself it was my fault
, everyone felt sorry for him and were very hostile with me. I admitted my part in the failings in our marriage, I had counselling, took the tablets and changed, he refused couples counselling and wouldn't even try to save our marriage. He gave me 3 weeks to get better and moved on, this is so painful for me?
Moving on two years later, he has divorced me and put vile, untruthful things in the petition. I came out of the marriage with absolutely nothing. But I've got myself a lovely home, my daughter is a delight, I have a good secure job, great friends and supportive family.
Over the weekend he called and was overly nice on the phone (things are usually frosty). He told me he had something to tell me, he has a girfriend and when our DD goes for a weeks holiday at the end of the month he wants her to meet her, and she is keen to meet her to!
So I ask what her name is, has she got kids, simple questions and he said it was irrelevant. We made an agreement we would discuss introducing other people to our daughter. I became suspicious and finally got him to admit it was her and he has been seeing her for ages! I can't describe how bad I feel, he lied to everyone, made me feel terrible and everything now fits into place, his lack of interest in his daughter for the last year, letting her down, saying he couldn't afford to see her, but went on holiday (told me with his mates, but obviously with her).
She has been introduced to his parents, he has lied who she is and I think they are shocked now they know it was her all along! But I feel absolutely gutted that everyone has been led to believe it was all my fault, and he was whiter than white. He still refuses to admit any part in the failure of our marriage, but I've become consumed with trying to make him understand ... why am I doing this?
I don't want him back he is not the man I thought he was, I'm happy on my own, so why am I sat here mulling things over, feeling terrible again? I've told him that woman has no right to think its OK to meet our DD after what she did, he thinks I'm being unreasonable. I'm not saying she can never meet her, but DD is only 4 and still doesn't understand why me and her daddy are not together.
It makes me feel physically ill the thought of her being involved with my daughter, and I am adamant she won't be meeting her? But can I enforce this, I don't think he would do it as he knows DD will tell me?
This has brought all the hurt, sad feelings back, so I need you to talk some sense into me!