The proper and respectful way to cancel a date would be to say - for example - “look, I’ve thought this through and I am not feeling it, I changed my mind about further dates, I appreciate the time you spent with me & I wish you all the best!”

The red flag here is mentioning someone “promising” and not wanting to miss the opportunity. It says in the same words “you are not a promising character”. The comparison is the hurtful thing you don’t say out loud if you want to be respectful of someone’s feelings.