I have so been there! Here's what helped our drama king when he was 4yo.
When he would fall apart, before I'd react, I'd take a breath and remind myself that I will not teach him self-control--which is my goal--if *I* flip out or lose my cool. Remaining calm myself was my first priority. This was the hardest part of my whole process, by far! But above all else, I worked to model an even keel.
Next, I'd ask him--mid-drama, mind you, often over something dumb like his sock being wrinkled in his shoe--to tell me on a scale from 1-10 how bad it felt to him. I defined this for him as utter misery, the worst feeling ever possible as a 10, and feeling perfectly fine as a 1. Inevitably he would sob "It's an ELEVEN!"
Then I asked him to use the 1-10 scale again, but this time to define how serious the situation was, with a 10 as something really horrible and not fixable that he could relate to (like death, since our cat had recently died. He favored the death of the whole world as the worst...), and "nothing wrong" as a 1. He started with 11 here, too, but when pressed ("Is your sock as bad as death? Really? Can we fix it? Can we fix death? Then maybe this isn't an 11..."), he quickly came to admit that a wrinkled sock was not as irreversible or painful as death. He usually came down to a 1.5 or 2. Then we talked about not reacting so strongly to a 2 but "using our words."
I used the system faithfully every time he got disproportionately upset over anything. It has two benefits that I can easily ID: 1) it breaks his chain of upset immediately, throwing him into analytical mode so he stops his weeping and wailing, and instead, he thinks, and, 2) it teaches the sense of proportionate response that he needs *so desperately* to learn.
I saw an improvement in his reactions the first time I tried it. He was much more easily calmed. It did take persistent use to reduce the drama to zero, but it really did work. Within a couple/3 months, he was able to tell me calmly that his sock was bothering him. No tears at all! And he was proud of himself for his self-control. I think it is very hard for a kid to feel at the mercy of his emotions all the time. If every upset is a tragic emergency, life must be a very hard thing. He seemed much happier to be able to control his reactions.
I don't use it very often anymore, but every once in a while, I still need to pull it out for him. He's 6, and it still works.
Feel free to ignore. All kids are different. But it was a GREAT system for our family. Everything got so much easier when he wasn't falling apart all the time!