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so what should I say, how do I cancel it?
Tell him that your plans have changed. If he persists for a second date, tell him that you are going in a different direction, and that he is not in your plans right now, or if he persists, not in your plans for the future. You can let him know that you appreciate his interest, but that you are looking elsewhere.
okay thank you :) what do I say when he asks me why my plans change? should I think of an excuse or just tell him I‘m not interested?
I'd say my plans have changed. If he leaves it at that, then good. If he persists, and wants to set up a future date, I'd tell him that my direction of interest has changed, and that you are no longer interested.
because there are aspects of his personality that I just don‘t like
Well if they are deal breakers then kindly pass on the date. If not give him a shot and worst case you learn something new about yourself