I often had found myself comparing my first ex with my second ex. I would often think, "if only my second ex did this like my first ex did" or vice versa. It was always a thought process that was going on in my mind. Maybe I didn't get closure from my first relationship like I wanted, or maybe it was because the first relationship was the first time I was romantically intimate with a guy. They say you never forget your first time, and I surely haven't.
I was given the suggestion that maybe I was just in love with the memory of that relationship and how it impacted me. Such pleasant emotions and experiences tend to stick with us for a long, long time. Perhaps you're just in love with the memories from that past relationship? If so, just know that it ended for a reason and you have a new love interest in your life now. Maybe use the wisdom gained from that previous relationship, what worked and what didn't, to help foster a better relationship with the girl you're with now. Just a suggestion.
I hope you can find peace and move on with your life.