She was your ex. You guys were broken up. She literally had zero obligation to not date someone else. You were both single. If you were holding out hope of reconciling, that's on you and you alone.

Take all of that personal growth you've accomplished and move on, either alone or with someone new (eventually). Don't be mad or bitter, and FFS don't pretend she owed you some kind of sexual fidelity when your relationship was officially over and had been for months.