Cheating on a good guy like Eric makes you a horrible person and some people may have even thought that he was starting to change you but then you cheat. He wa turning you into a better person and I think you didn't like that and you wanted to stay the same so you did the worst thing ever. Cheat.
All I can say is: mistakes shape who you are. Yeah, I made a mistake, but at least I own up to it. And as for me "changing" that's me as a normal person. Nobody can really change, even for a relationship. What you saw when I was with him was the real me. Hard to believe, I know, but that's the truth. If you refuse to accept that, sorry.