How can one honor his deceased mother and gain her pleasure and God's forgiveness after having been disobedient to her during her lifetime?

You must offer sincere repentance for disobeying your parents, and increase your good deeds, supplications, and prayers for forgiveness for your mother, as well as giving charity on her behalf, honoring her loved ones, maintaining ties with her kin, and paying off her debts. All of this is considered righteousness towards them after their death, and perhaps Allah will alleviate, through these actions, the previous disobedience. You must also beware of emotional outbursts.