I resent my gf for having an abortion
Why would you resent her for that?
I believe that killing an innocent human being is immoral
I can understand why you feel that way.  Having a partner make a decision like that without your input can be very frustrating.  However, you need to respect the fact that ultimately it was her choice, not yours.
I can't change my feelings and you can't force me.
Unless you both agreed to have a child beforehand, I don't where you have standing in her decision. I have no intention of forcing you to believe anything one way or the other; likewise, you shouldn't want to force her to go through with an unplanned pregnancy.