Today I didn't know I needed to wear a mask and nearly made myself have a heart attack. [I Social Anxiety]

Forget about it. I've had the second vaccine and I keep forgetting one, meaning I leave the house then have to run back up stairs to grab another. My neighbours probably think ugh weirdo...
I couldn't care less.
I have been in a shop without one and I ran in grabbed what I needed then self checked out really quick.

There was one staff member and one other visable customer. I think it was OK.

Bottom line, you made mistake, this is a stressful time, they may think you're exempt.

I don't know why you thought you didn't need to keep wearing a mask, but you are hugely overreacting. If this is the way you usually respond to life, to minor negative experiences, you really should be in therapy. You have significant mental-health issues. Please seek professional help.

As for masks, just carry one or two with you, in a wallet or purse, wherever you go. That way, should you forget to put on a mask before you leave the house, you'll always have one with you. I've occasionally forgotten, but because I have extras in my bag, I've been able to pull one out and put it on when I've realized I'm maskless.

What people forget is though there are 90 something million Americans who are fully vaccinated, there will be just under 5 million of them who the vaccine does not work on. They don't know who they are neither does anyone else. They can easily pick up this more transmissible version of the virus and give it unknowingly to a bunch of people.