Self-regulation is a plain thing with a clinical name: noticing where your body and your feelings are, then doing something about it. Settling down. Waking up. Waiting one more minute. Adults run it all day without noticing. It is a skill that gets built, not a personality, and it gets built on top of the sensory system. If a kid cannot feel his body revving, he cannot bring it down. If the world already sounds too loud to him, he starts every morning much closer to the edge than the kids beside him.
Which is why the meltdown is almost never about the cup. The cup is the last small thing stacked on a day of noise, transitions, hunger and effort. A consequence does not install a skill, and “calm down” only works on a kid who already knows how. So we build the pieces in order: awareness first, then strategies he can run without you standing there, then practice under real pressure. A comprehensive initial evaluation is where we find out which piece is actually missing.