Is talking to oneself about a person who angered you considered backbiting, and does it entail sin? And how would one repent if that were the case? Or does backbiting necessitate the presence of more than one person in the conversation?

The fundamental principle regarding belittling people and mentioning their faults—whether verbally or in one's heart—is that it is a bad characteristic. It is forbidden to think ill of a Muslim. Whenever a bad thought about a Muslim crosses your mind, pray for good for them. And whenever you learn of a Muslim's slip-up, advise them secretly. However, if secret defamation is justified, such as to repel the injustice of an oppressor or the aggression of an aggressor, then there is no sin in it, based on the Almighty's saying: (Allah does not like the public utterance of evil except by one who has been wronged). Nonetheless, it is preferable to refrain from reviling and instead occupy oneself with what purifies the heart.