This is getting REALLY annoying. My Sims keep cancelling their actions and it makes me throw my monitor out the windows! It's not me, it's them doing it by themselves! HELP BEFORE IT'S TOO LATE!

HAHAHHAHA no. I'm using NO mods.
Is it a glich or what?!

Their mood levels are probably too low, or it's a glitch. If you go into game settings, you can reduce their autonomy, which is their freedom to act as they please. If that doesn't work, you can try "Force Error > Reset." Otherwise, I have no idea. It is frustrating, I know! Sorry if you know all this already.

Thanks for the help! :D

Without cheats or mods, my Sims are often terribly tired. That's probably the most severe ongoing mood problem I see in them, and it is a reason for their abandoning an action from their queue. Good luck to you and your people.

Your sims have gained sentience and no longer desire to follow your orders; nuke them now before they find a way out the game and kill you. Lame jokes aside, it's like everybody else said, they're either too low in motives or their autonomy is set too high

Also, if your sims have the absent minded trait, they often "forget" what they were going to do.

Teens undergoing a mood swing aren't well known for doing what you tell them to do. In the game, either.