Hi, 
It has been decades, but what can I say, I’m a slow learner.  I need you to know a few things that I was unable to say at the time.  
I was young, immature, and emotionally stunted during that time in my life.  You were a perfect partner, and friend, and I treated you poorly. When I ended things between us, I did what I thought was best for us both by cutting ties.  I was confident it was over at the time, and I didn’t want to string you along, and make you feel that there was a chance that we would get back together. I thought cutting ties was the best way to do that.  It’s only now, after 20 years that I realize how wrong my actions were.  I regret my actions, and am deeply sorry for ever having hurt you.  
I heard you have a family now.  I wish you and your family success and happiness.  I’ll always make myself available should you ever want to reconnect.