“Feelings” are not always the best gauge of reality. Reality is that there were reasons you broke up. There were reasons it didn’t work. There were reasons you dated “for years” and yet did not move to marriage.
Go back and think about them.
And, you are reading something in to that-- some hesistation on her part regarding her future spouse. You are basically making up a story in your head to explain this fact. You could have the wrong story-- maybe she is perfectly fine with her intended and thought you would be devestated or something.
You are putting some big meaning into this that likely isn’t there.
Again, you are creating a story in your head.
This ship has sailed. I would let it go and cease contact with her.
Focus on YOU and how to be the best person you can be. Examine why you are still clinging to this old relationship, and now-- only after she has declared her engagement to another-- do you feel moved to pursue her.
Get some counseling.