In the moment, you accepted the invitation because it seemed like a fun time. The time has come and now you'd rather pull your fingernails off one by one than go out into the world. But if you change your mind, now you're a flake. Now they are going to think you're a stuck up bitch that never wants to see them. Now you feel like shit for cancelling plans at the last minute or just blowing them off completely. Now instead of staying home and relaxing like you told yourself you needed, you are sitting in silence going over future conversations in your head where you make up excuse after excuse on why you didn't show up.
So, how can we avoid the anxiety of cancelling plans because we simply don't feel like it? The first thing you need to understand is that you are not obligated to do something that is going to make you uncomfortable simply because it would make someone else happy. Now repeat that again.
Nothing about this act is selfish. As someone who struggles deeply with social anxiety, whether it be that being around other people seems incredibly draining or I simply think that my hair is too flat to be seen in public that day, choosing to not put myself in a position that could make me feel terrible isn't selfish.
This is equally as effective when it comes to initially saying no to something or someone. It is okay to say no. I live by a simple belief of, "If it's not a fuck yes, it's a no". What this statement means is that if you aren't 100% for something, just say no. If someone is offering something you don't particularly want, saying no isn't selfish. Even if that person means the world to you, you aren't obligated to please them.
Sure, you may experience some aftermath from that but if you explain yourself, explain that it wasn't something you wanted, explain that you were simply just too drained, and are honest and transparent with them, they can either respect your honesty or they can't. And honestly, if they still want to be upset, let them be upset. That is their problem. You need to take care of you. And sometimes that means saying no to everyone but yourself.
And that's not selfishness, that is self care.