How can I atone for wronging my professor by calling him offensive names and writing terms that insult his person, after I realized that the reason which drove me to do that was not justified?

Allah has forbidden finding fault with people, reviling them, and mocking them. Allah Almighty says: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them." And the Prophet, peace and blessings be upon him, said: "Insulting a Muslim is wickedness." And it was mentioned that "every Muslim's honor, wealth, and blood are sacred to another Muslim." Ibn Kathir clarified that mockery is belittling and ridiculing people, which is forbidden, for the one who is belittled might be better in the sight of Allah than the mocker. It is obligatory to repent to Allah by ceasing the sin, feeling remorse, and resolving not to return to it.