I want my girlfriend to keep a baby that she wants to abort
You want the child yourself?
I want the child. Even if my girlfriend doesn't, she could just give the kid to me.
It's fair enough that you want to protect the kid. It's also fair that you'd want the kid even if your girlfriend does not.
I want to be a good dad to that kid.
I am sure that you do. Just try to understand that this is ultimately the woman's choice.