Why is it so hard for me to be social?

I was in a relationship for almost five years. We were friends for a few years before we got into a relationship. He was a caring, friendly, and charming guy up until three and a half years in. He started controlling me: who I hung out with, where I was at all times, and he had a huge hissy fit if I tried to ask him the same questions. All that time, he just liked me as a friend and was cheating on me with my so-called friends. At the end, he was a nasty and narcissistic person. Now it's hard to be social.