How much trust do you place in those you call friends?

I have been betrayed by people who I called friends for Years . In some cases I have discovered they have always been jealous. They spoke ill of me to mutual friends . They have tried to use me . They become angry with me about insignificant issues. They never explain why they are angry.
I never attempt to get even; however when people try to hurt me I will defend myself. I think this is a realistic attitude .