Dear Sister Mary Martha, I'm really tired of being nice all the time, and I'm ready to snap and say something mean.
Snap out of it.
What gives you the right to be mean to anybody?
Is everyone else stupid? Is that it?
I have news for you. They're doing the best they can. Yes, they are lame. Maybe they do need to try harder. However, if you snap and say something mean, they won't for one second think to themselves, "Wow, I'm so stupid." What they will think is, "what a mean person." What's the point of that?
The world is a difficult, often terrible, place. You have a choice to add to the terribleness of the world and make it an incrementally more terrible place or to try to make it a less terrible place. You can add ugliness or you can add beauty. You certainly won't help one blessed thing get better by being mean.
What you will do, besides degrading the state of the world, is make yourself into a bitter, crabby person with some sort of sense of entitlement that things should go better for you.
That person that you want to be mean to is a whole human being, too. You've lost sight of that.
On top of that, if you had simply snapped and said something mean, it might not be a sin, because you may not have had any actually intention of being mean or hurting anyone. There was no intent, just a momentary lapse, for which you could have apologized. The end.
But now, you're giving yourself permission to give into your anger. And that is a sin. Anger is not a sin. Living in anger is.
Perhaps you need a break. Put your feet up with some tea and take some deep breaths to quell your anger.
Whatever you do, the idea that you are simply too weary to be nice is an idea to be dumped in the trash heap of useless plans. Get some sleep. And some gratitude. And some compassion.