Based on your question it sounds like you are not sure about the person you are engaged to marry. If you knew in your heart that you had "found your soul-mate", "the person you want to spend the rest of your life with", "the person you can't imagine living without....etc" There is no way you would have any concerns about what your EX wants!
You can simply tell your ex you appreciate everything the two of you had in the past but you have moved on and found the person you want to grow old with. Let your ex know you are hoping they too will one day find their soul-mate.
Always remember an ex is an ex for a reason and it's usually a very good one! Your future lies ahead of you and not behind you.