A "Dear John letter" is a letter written to a husband or boyfriend to inform him their relationship is over, usually because the author has found another lover. Dear John Letters are often written out of an inability or unwillingness to inform the man in person. The reverse situation, in which someone writes to his wife or girlfriend to break off the relationship, is referred to as a "Dear Jane letter".
I'm writing to inform you that I've found another replacement to my lonely vagina, please understand that, I'm a whore and needed another man.
(Dear John Letter)