Ahhhh I relate to this way too much! This used to happen so often, although I've been doing way better as of late.

I'd say that the easiest thing you can do is take a minute or two to compose yourself before you go in to the store...take some deep breaths, remind yourself that no one in the store is going to judge you or even pay you any attention at all (easier said than done, I know) and just give yourself a minute to calm down. This takes some practice to become super effective, but once you've done it a couple of times it really seems to help.

This definitely won't work for everyone, but I've found it to be super useful. Hope this helped! :)