I have just become a father as well, and I am not in a good relationship with the mother yet we are forced to live together and I am supporting her 100%. We argue daily, she blames me and I blame her. She has never done things that your ex did that is very heartless and will come back to haunt her someday in someway. Having a child is life changing as you know and a lot of guys in your shoes would be gone. There is not equal rights when it comes to children it is the coldest of truths. If a man slept around on his pregnent ex he would probably never get an opportuntity to see it as a baby. Truth be told your main concern should be your child and whether her behavior affects your child. Someday your child is going to be older with the abilities to think and act on their own behalf, they will make their own decisions about relationships and they won’t remember what happened as a baby. They might see pictures of events minus you, but in their maturity they should be able to see that it that from the time they can remember you have always been their father who puts them before all else where as the mother can not say that. It is a shame you missed that stuff and the way she is being and you and her are always going to be connected so you need to make her realize that. I believe you can get a DNA test to prove you are the father in order to get rights, (or maybe you aren’t even the father) but that won’t change mom’s behavior. I really can not believe she was able to do so much with a newborn baby as they need constant care. I am not an expert but I would say you should talk to a lawyer as soon as you can, if her situation changes she may come after you for back child support + support from there forwardwhich will exceed what it would have cost had she included you. As for the bitterness, the only thing you can do unfortunately is chalk it up to a bad situation and try to make it right by the child that is be there, and realize the mother is who she is, she sounds rotten but as long as her behaviors don’t affect your child then let her go.