Okay, just pause. Now take a deep breath. Okay good, now another one. One more. Don't you feel better now? I do.
Note for the day -
None of the little things can bother you. They are as rain is to a duck. They will just roll right off of you, unable to affect you. Don't forget that it's easy to be nice to a pleasant person, but it's a mark of true character to be nice to a difficult one. Even if they make you want to puncture your own eardrums with a rusty bicycle spoke so that you don't have to listen to them. Even when you literally have to bite your tongue to keep from saying something unkind.
Now a couple more deep breaths. Breath in. Now out. Inhale. And exhale. Now you're ready to be polite again, if you can't avoid them for the rest of the day.