In love with the one you can't have

I recently went to a school reunion night out, at the event was an ex-girlfriend from 20 years ago, the moment our eyes met, it was like we were teenagers again! We had a moment at the end of the night, which lead to other moments, and essentially us declaring our undying love for one another! The problem being, that she is with someone and she has 2 children with him! We have been in contact with each other since the night, for 4 months now, messaging every night on fb and having secret meetings as and when we can. She has said that I am her "Achilles heel" but that she doesn't want to leave her man, because of the kids! I know she is right in keeping the family together, but it leaves me feeling lost, confused and incredibly low, I spend hours sitting by my phone waiting for a message from her, sad I know, but I'm completely smitten by her, the feelings are so strong, I cannot explain! She has told me that she is not happy in her relationship, even has gone so far as to tell her other half this, but he has stepped up his game and she is determined not to upset her kids! I need to move on and forget her, but it's destroying me from the inside out! Please help!!