Your mother is in God’s hands now. If she is in heaven, she would have long forgiven you, for there is no unforgiveness in heaven. If she is in purgatory because of unforgiveness, then you must pray for her and make reparation to help her.
As for you, what you need most is to go to confession. Tell the priest what is on your heart and let him help you. But most of all believe that God has the power to forgive you and to set free both your mother and you. And you have to forgive yourself, for you are not greater than God, who Himself is forgiving you.
Don’t worry about needing your mother to know your regret. She would have seen it all when she faced God face to face right after her passing. God would have shown her everything and she would have made the choice to forgive you then.