You ought to beg her forgiveness and assure her that you would never do so in future. Hurting one’s parents is one of the gravest sins in Islam, second only to shirk or to associate partners with Allah. The Prophet (peace be upon him) said: "Shall I tell you of the three most grievous sins: He repeated the question thrice. Then he said: It is to associate partners with Allah; to displease one’s parents, and to bear false witness!” (Reported by Bukhari)
He also said, “Allah’s pleasure lies in the pleasure of one’s parents even as His wrath lies in that of one’s parents” (Reported by Mundhiri)
Furthermore, it is important for us to know that honoring our parents and treating them with utmost kindness is the second foremost commandment in Islam. We learn from the Qur'an that the second commandment issued to each and every prophet from Adam to Muhammad. We read in the Qur'an: "Your Lord has ordained that you shall worship none but Him. And do good unto [thy] parents. Should one of them, or both, attain to old age in thy care, never say "Ugh" to them or scold them, but [always] speak unto them with reverent speech and spread over them humbly the wings of thy tenderness, and say: "O my Lord! Bestow Thy grace upon them, even as they cherished and reared me when I was a child!"(Qur'an: 17: 23-24).
Therefore, I would urge you to continue to beg your mother to forgive you and continue to treat her kindly so that she would turn towards you with her mercy.