Loves of Future Past

I already know this type of problem has been posted before but please bear with me ...

I dated this girl in high school, over 20 years ago now, and like all teenage romances, I broke it off with her due to a little "birdie" chirping in my ear rumors that I eventually found out much later were not true, all just so that the "birdie" could start dating her. Since then, the girl and I have never really kept in touch due to both of us moving away from our hometown. We've talked briefly in passing to each other when we would encounter each other in public on visits home but never really talked. We had even "Friended" each other on Facebook, although I rarely use it.

Well, after 20+ years and two failed marriages (one I was in love with and one was a rebound marriage), I decided to move back to my hometown and to take some time for me since I really hadn't been single since my senior year in high school. I was single for a good 18 months, but over the past 2+ months, I have gotten into a relationship with a woman that I absolutely adore and could see a future with her.

However, going back to May of this year (2014), I saw a post on Facebook of my "high school sweetheart"'s and I commented on it. She commented back and the next thing we knew we had made plans to go out and eat and catch up, because we both found out we were living back in town.

During her 20+ years since dating me, she had been married (not to the "birdie") and been in a few more relationships, had a couple children, and like me ended up back in our hometown. But ... when she moved back to our hometown a few years ago she started dating the "birdie" again (and currently lives with him)!

We played it off the whole night as if it were two friends just catching up, but I could feel the old feelings coming back, not only from me but also from her. After several hours at the restaurant we parted ways. After this we went back to our not keeping in touch except the occasional comment on one of our Facebook posts.

Then, on Monday, out of the blue, I get a message on Facebook from her. It was regarding a post she posted regarding "If you could write a note to your younger self, what would you say in only two words?" On her public post, she said that it was too many things for just two words. The private message to me, however, had the words "Keep [my last name]".

After messaging for a few minutes and trying to figure out what she meant, we both realized that the feelings were still there and as strong as they were 20+ years ago. We realized that the other was "the one that got away".

Over the last few nights we've been talking and reminiscing and getting to know each other again. We've realized that we both want each other but her in her relationship and me in my relationship, we don't want to hurt the one we are currently with, but we can't deny the feelings for each other much longer.

Being in this territory is new to me. Any advice would be greatly appreciated.