I'm going to cancel long-term plans with my ex last minute.
Why do you want to cancel long-term plans with your ex?
It's a pity to use a douchebag like him for a long-term plan.
Not wanting to hang out with an ex is normal but you shouldn't cancel plans at the last minute. Even an ex deserves the respect of letting them know ahead of time so they can make other plans.
What if I hate him and he is jealous of me.
Then why are you hanging out with him at all? At any rate, don't cancel at the last minute.