What is a name for a friend who criticizes you too often?

One possibility is: former friend. Another is 'critical'. Tell your friend that the criticism is coming a little too frequently and you'd appreciate a break. If it just keeps coming, then spending less time with that person is not a bad idea. No one likes to hear non-stop criticism. If the person really is your friend, do bring this up. It seems it is a fair criticism that your friend really needs to hear.