Acceptance means allowing yourself to acknowledge what is happening, including how you feel about it, so you can decide how to respond.
You don’t have to like the fact. You don’t have to approve of the person who caused it. You don’t even have to feel calm while admitting it.
You can stop arguing with the fact without agreeing to the arrangement.
They cancelled again. You understand the reason. You also wanted to see them, and being understanding hasn’t made you less disappointed.
Calling yourself ungrateful adds an argument with yourself to an evening that already hurts. Acceptance lets the disappointment be there long enough to ask a useful question: what were you hoping to receive, and what keeps happening instead?
Sometimes the next step is a conversation. Sometimes it is making a different plan. Acceptance can make a refusal possible; it doesn’t require an endlessly accommodating yes.
When acknowledging the hurt becomes another reason to attack yourself, self-compassion offers a kinder starting point. Choice looks at what remains yours to decide once the facts are on the table.
Russ Harris’s The Happiness Trap worksheets (PDF) offer practical exercises in acceptance and values. Read them for the difference between making room for a feeling and letting that feeling make every decision.