Everyone don't do it, I know, but if someone does mistake and comes forward to make it up, he deserves some leniency.
If I were u I would pay the child support and leave the GF or try to abort the child. If she does gets the abortion, live double life for a while then leave her after sometime. You don't have to tell your wife. Everyone has some secrets, its better if you move forward and be honest after learning this lesson.