What is the religious ruling on ending a friendship with a friend who always assumes the worst after years of companionship?

It is not permissible for a Muslim to harbor ill thoughts about his Muslim brother, due to the Almighty's saying: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." And the Prophet, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the falsest of speech." If ending a friendship means cutting off ties and shunning, then that is not permissible for more than three days, due to his saying, peace and blessings be upon him: "It is not lawful for a Muslim to desert his brother for more than three days." However, if the intention is to lessen the closeness of the relationship, there is no objection to that.