My Similar Story - A Happy Ending
Here's a bit of a true story that might, I hope, help a bit.
A long time ago, I had a best friend named Christopher myself, who was also one of my D&D player friends, and a virgin who had never dated anyone. I also found my first real, long-term, adult relationship with a woman. My girlfriend also cheated on me, and left me for, Christopher. It broke my heart, and sent me into a depression for a year.
Worse, while we were going out, Christopher became her best friend, and manipulated things to get us to break up (things like always bring up arguements we had, focusing her on the negative all the time, making sure she was too busy to see me for important events, that sort of thing). Christopher ended up dating her for a longer period of time than I did.
Only thing is, the story had a happy ending. Christopher and I eventually became friends again. We shared stories about this woman, and laghed about similar events. He told me how he had executed on a plan to break us up.
Eventually, he broke up with her himself.
Christopher is still my best friend, and we rarely, if ever, mention the woman anymore. When we do, we both laugh about the fact that either of us ever were interested much in the woman (not that she is evil, just controlling and not either of our types), and how stupid it was that we let her get in the way of our friendship.
I'm now happily in a relationship with a woman, as is Christopher. And we still game together (though he no longer plays D&D).
I'm not suggesting you run out and become friends with your Christopher right now. I am saying, however, that time brings perspective. You never know how things are going to look, down the road.
Generally, the rule is that it takes about half the amount of time you were with someone, to get over that person, and obtain your perspective. That means you have a rough 2 years or so ahead of you, probably.
Also, though this would be much more difficult with a child, I have found it best to have as little contact with your ex as possible, until you are over her. Unless it is for the benefit of your child, don't try to be friends with her (or enemies, or anything). Don't re-read old love letters, don't stare at pictures of her, and don't fantasize about her. Be a father to your child, but don't interact with your child's mother any more than you must, for about two years.
How you behave now will have an impact on your child. Make your little one proud of you. Have patience, and you will obtain perspective on this.
That's my advice, for what it's worth.