I'm going to tell my girlfriend that we're done if she doesn't want another kid in the future
Why would you say something like that to her right now?
She is extremely reluctant and this makes me angry
I can see this is an important issue to you but you should not give your loved one an ultimatum. This will make them angry and resentful.
She's just selfish. I'm going to end this if she does not want to have another kid.
I understand that you want to have another child and it's your choice whether or not to stay with your girlfriend, but I really don't think you should force her to make this big of a decision right now. Giving her an ultimatum isn't going to make her change her mind about having another child.