You've probably heard the old saying that "Two wrongs don't make a right" -- and it's true, especially in this case. What you did by having sexual relations outside of marriage was wrong in God's eyes -- but allowing the little child that is growing inside your girlfriend's womb to be killed would also be very wrong in God's eyes.
In other words, don't ask God to bless something that is wrong -- which is what you'd be doing if you prayed for Him to change your girlfriend's mind about an abortion. Does this mean you shouldn't pray? No, not at all -- but the prayer you need to offer is twofold. First, you need to offer a prayer of confession, asking Jesus Christ to cleanse you of your sins. Then you need to offer a prayer of submission, turning your life over to Christ and asking Him to guide you and help you do what is right.
I hope you won't think I'm harsh or judgmental; that isn't my intention at all. But you are standing at a major crossroads in your life -- and I pray you won't take the wrong road. God loves you, and your greatest need is to discover this great truth. Jesus said, "I have told you this so that my joy may be in you and that your joy may be complete" (John 15:11).
You have some hard decisions ahead -- but isn't it better to face them with God than without Him? Don't be pressured into doing something wrong or unwise, but ask God to help you find the right path for your child -- through adoption, for example.