Rectifying one's relationship with one's mother after her demise

My parents are no more. During my mother’s lifetime, due to various reasons, I was sometimes harsh with her. I used to shout at her and speak rudely, and unfortunately I could not ask for her forgiveness while she was alive.

However, during the final stage of her life, around two hours before her death, she became very ill and I took care of her sincerely. At that time, she gave me many duas. But throughout my life before that, I regret that I was often rude to her.

Whenever I pray Salah, I constantly ask Allah for forgiveness for the way I behaved with my mother. Even after her death, I once saw her in a dream. In the dream, I was saying to her, “Please forgive me, Mom. If you do not forgive me, Allah may also not forgive me.” She did not say anything and was only looking at me.

Please guide me on how I can seek forgiveness properly and how a person can know if Allah has forgiven them for such a sin.