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You cheated on him and yet you're wondering what he thought you were to him. You can't blame it on being 20 and young. I'm sure a lot of the 20 year olds on here would tell you they wouldn't cheat. I know when I was 20 I wouldn't have cheated. You can't use age as an excuse for that.
Also, it's been a while since your relationship with him. He was married to her and he has kids with her. He obviously loved her if they were together for 13 years. Yes they separated, but obviously he loved her enough to want to try and work things out with her. I doubt when he got back with you temporarily that he felt any love for you. It was probably just a fling to him. Perhaps even being with you made him realize he wanted her back.
I fully support a man who wants to get back to his wife and kids. It is her he should be with, not you.
Also, you can't have always been in love with him if you cheated on him. You asked: "What exactly was I to him? Did he ever love me?"
You should be asking yourself those questions when you cheated on him. What was he to you? Did you actually ever love him? Doubtful if you could cheat on him. It's time to move on.