Is he who is accustomed to cursing a person by saying, "May Allah curse you and curse whoever created you," considered an unbeliever, given that he realizes the last part comes out unintentionally, and his intention is only to curse the person, and then he seeks forgiveness afterwards?

Abusing Allah is an act of disbelief that expels a Muslim from Islam, by scholarly consensus. As for the tongue becoming accustomed to disbelief, the determinant factor is the intention behind uttering the words. So, whoever intended to utter it is accountable, and whoever did not intend it, meaning the words merely slipped from their tongue unintentionally, is not accountable. This is known as laghw al-lisan (slip of the tongue). Thus, whoever intended the speech and the abuse is a disbeliever. And whoever intended the speech that indicates abuse while joking is also a disbeliever. However, whoever did not intend the speech or the abuse, but rather their tongue merely slipped, is not accountable. As for merely accustoming the tongue to forbidden words, this is not an excuse for avoiding accountability, and one must repent from it.