It took both of you to make a baby, so its both of your responsibility to raise him/her. You cant force her to have an abortion, its her body, her choice. I think you should stand by her give her support if she needs it, play an active role in your baby's life, be the best parent you can. Walking away from your baby is weak and pathetic just because it doesnt fit in with your life and your new relationship get over it, you're 21 you should be thinking like a man not a selfish boy. If you do that, im sure one day your child is going to come looking to meet his/her father and be very angry and hurt that you left them. You owe it to your baby to be there for it.
A lot of girls/women tell guys they are on the pill when they arent, because they want a baby. Im not saying she did that, but you should be more responsible and use condoms instead of leaving it upto the girl to use protection.
I dont know how your mum can just brush it off like that when its her grand child, shes awful!