You’re allowed to change your mind about a plan. Just say so early and honestly.

Changing your mind is not a failure of character, and pretending it is keeps people stuck in plans they no longer want. What people actually mind is hearing about it at the last minute, because the late notice is what costs them something. The decision belongs to you; the timing is what you owe them. Early and honest beats committed and resentful.

If something on your calendar has turned into a thing you dread, say so now rather than on the day. One plain sentence, no invented excuse, and another time if you mean it.