I was the result of sexual immorality too and have suffered through not having a father around as he has spent the majority of the past 19 years running from the responsibility.
Don't punish your child. If you can't love the mother as much as you ought to, at least get along with her. Don't insult her, don't beat her and generally don't do anything that would cause your child to disrespect you. Be a good father to your child and ask for God's guidance. He may change your feelings about this woman if he wants you to marry her.
Remember God will always forgive everyone who truly repents. I am sure that you will grow to love your child and in time will wonder how you ever lived without him/her.