How to stop being hurt when my friend cancels plans last minute?

One of my best friends has pretty severe ADHD and therefore is quite flaky, forgets things that get in the way of plans, or generally cancels pretty last minute. It doesn't help that their parents are pretty controlling and volitile! But I get really upset whenever this happens, even when I logically expect it, and I was wondering if you lot have any ways I can cope better with this. I try now to make plans with them in groups rather than just 1:1, because if they can't make it the plan still goes ahead, but I'm still upset at them not being there, and it doesn't always work.

(P.s. Sorry if this isn't the right place or I use the wrong flair, I feel like this fits into a few of the catagories! :) thanks!)