I'll call her Nancy

OK, here I go. Many years ago, I fell in love with someone.We both worked at the same place. From the first moment I saw her, I was completely taken. At the time I was married and had a young son. To make a long story somewhat shorter, one night I told her I was in love with her, and she said she was in love with me too, and thought about me all the time. Well.... I was willing to do anything and give up anything to be with her, and left my wife and son.(by the way, she also was in a relationship at the time and was living with her boyfriend). We would meet when we could, stolen moments in a parking lot, or at a restaurant where we knew her boyfriend wouldn't go. We never had sex. Close once , but no. that's another story....
Until one day, she said to me "I made a mistake" and that just crushed me. I got so depressed I couldn't function at work, so I got fired.
I never got over her.
Here we are 35 years later and who do I come across on Facebook? You got it. We start talking and all the old feelings come back. Again, I'd give anything to be with her. I'd walk away from my life now (I've remarried, got grown kids and live and work thousands of miles away)if there were a chance to be with her.
What's wrong with me? Why do I want his woman so much?
I know she doesn't feel for me the way I feel for her.
It's been 35 years for God's sake!
Help!