Have you ever been betrayed from someone who you considered as best buddy ? All the time, he was acting like everything is great but at the same time he was ambushing you behind your back . Have you ever been really disappointed in someone and how did you react on that betrayal ? Did you forgive him/her , if you did, have you ever again had full confidence in him/her like before betrayal ?
forget about regaining full confidence in him/her, something similar occurred to me also, and what I did was just move on in your life and treat him/her as a passer-by.