It's not you it's me
A phrase used when terminating a relationship. Usually used to confuse and/or draw attention away from the real reason the relationship is ending, because of the emotionally painful nature of the reason. 

Rarely used in person, more often used in phone converstions, e-mails and voicemails.
He was a nice guy, but he was as good in bed as a cement block. So I called him up last night and told him it was over, but it wasn't him it was me.