I am not autistic (I don't think, anyway), but have a host of other issues. I have the same reaction as you to social situations, and the suck of having to cancel things I enjoy because I just can't do more social at the moment. And the guilt of being unreliable because of it all. It took me a very long time to get up the nerve to do this, but if the above involves people I know well -in my case, usually coworkers - I am just super blunt and up front with them. "I am not feeling great / I'm not up to this, I'm sorry I won't be joining you / I'm going to head home now ". etc. I've done it long enough now that they are used to it, and they can see that when I am up to it, I do whatever it is I need to do. I just got sick of finding excuses when I was pretty sure they all knew it was bullshit anyway. It's had some positive results for me, too, in that they stopped asking me to do some things because they knew I was probably going to be burned out by then and nope right out of it later.

I understand though I am not a person with autism just introverted.