I haven't done a single thing for her to hate me other than the fact that he completely divorced her for me.
I'd say that may be part of the problem. You were seeing a married man, and he was cheating on his wife.
Second, you are not married to this man. Third, YOU do not have shared custody of his child, he has shared custody with the child's mother.
You sound very bitter toward his ex wife, with your comments. That attitude of yours, will cause the destruction of your relationship with your boyfriend. He is tied to her until at least, his obligations to his abandoned family are met with the children becoming adults.
So you have many years to go.
My advice to you is to realize your role is not to interfere with your husband's legal obligation to his child, and/or become involved with all conversations that take place between the two of them. Just step aside and let them deal with the business of making decisions together, talking together, arrangements, etc. and realize that it has nothing whatsoever to do with you.
If you choose to stay, the longer term will hopefully see you putting the children first, without resentment, or expecting anything to come of it. It won't be any easier if you marry this man, and be careful about that too- if he cheated on his wife, and left her for you, in my book, he's not exactly a stellar guy to start with.
Try not to characterize her as being a horrible person, as you have. Maybe someday you will know what it feels like to have your husband leave you for a woman who is 14 years younger. You started dating him when you were 22 and he was 35.
If you can't live with what you have created, then yes, it's time to get out, and leave any future married men you meet, out of the picture.