One of life's hardest lessons is that we can't go back and change the past. Much as we'd like to, we can't undo the wrong things we did, or (as in your case) do the things we failed to do.
All we can do about our failures is seek God's forgiveness which I hope you will do. Let me be clear: What you did was wrong in God's eyes (no matter what the circumstances were). Your mother needed your help, and your relationship might even have been healed if you had been willing to give it. But God still loves you, and Christ shed His blood on the cross for you so you could be forgiven. Receive His forgiveness today by turning to Christ and asking Him to cleanse you and come into your life.
We can't change the past but with God's help we can change the future! The Bible says, "I know the plans I have for you ... plans to give you hope and a future" (Jeremiah 29:11). Ask God right now to help you be the mother to your children that He wants you to be. Let them know you love them, and do all you can to encourage and help them. Learn from your past, and make sure you don't repeat the mistakes you and your mother made.
Most of all, pray for your children, and ask God to help you point them to Christ. Someday they too may have children, and your example can help them be the parents God wants them to be.