Your heart longs for what has been denied to you throughout your abusive marriage.
You've devoted yourself to the lives of your children and possibly neglected your own emotional needs. Always being there for other's is a distraction from your unhappiness.
Maybe this former flame feels like a safe place to release these emotions as you will never meet. Nurturing this fantasy is form of escapism for you but your heart has been caged for too long and the tantalising thought of feeling loved is torturously out of reach.
Abusers train us to always put their needs first whilst making us feel invisible and invalidated. I think you have a lot of love to give but when did you last give any of that love to yourself?