Never get jealous when you see your ex with someone else, because our parents taught us to give our used toys to the less fortunate. Yup, i need to remember that. Don't worry Katie, you'll find out soon enough that you are the less fortunate now.
The person might not be who you thought they were; might have changed for the worse; but in the memories, they will always be the person you loved. it's ok to cherish those memories. You dont have to allow that person in your life now.