Try to stay single for a while after you break up. Because if your Sagittarius ex happens to see you with an immediate partner, he might come to the conclusion that you set it up all along and blame you even more.
Tell her you cannot keep up with them sexually or physically anymore. Tell her she has too much energy and it is hard for you to stay at the same level. She needs a partner who can keep up, so she will understand this reasoning.
Act bored. Just slump on the couch and slow down. Become a homebody. Sagittarius likes to be out and about, and he gets involved in all sorts of activities. He won't want to be with a boring person.
Act insecure. Sagittarius's love confidence and if you act all paranoid or insecure about yourself and in the relationship, she will definitely step away.
Show signs of jealousy and possessiveness. Sagittarian signs cannot stand this trait in someone. Ask him where he was, and with who and why he went there. He will be out the door.
Lose all interest in sex. Tell her you want to be celibate for a year. She will be gone in a flash.