Except if she wasn’t sure she didn’t want to settle down...she shouldn’t have. We didn’t have kids until after we were married. And even still I’m younger than her and I prefer being “settled down” or whatever you want to call it. She betrayed the commitment we had without ending it first. She tried to have her cake and eat it too.
As you have said, she had a void that needed to fill and a very difficult past to deal with. Being sure about what ones want is not that easy in that scenario. She probably got engaged with you hoping that you could fill that gap or simply because it could bring some stability to her life.
I don't blame you for having tough thoughts about her. You'll need a thick skin to get out of that relationship but it's not as simple as saying "if she wasn’t sure she didn’t want to settle down...she shouldn’t have".