Doubts in a relationship

I'm sorry if this sounds absurd, I don't know what to do with myself. I struggle with anxiety, depression and I have trust issues as well. I'm with this wonderful guy, everything has been okay, we haven't really have had any fights but, before he dated me he used to like tons of girls and then one night he says "I used to be one of those guys who liked many girls at the same time, but when I started dating you that changed" and I didn't believe him, I know I should but I don't know how, what was that "changed him" that anybody couldn't. I'm no magical person, I'm just an average girl, help please.