Hi, not the person you asked, but I can kinda answer from my own personal experience if you don't mind. I'm a lot better now then I was a year or two ago, (still have some lots of improving to do though) and not as bad as the person op was describing, but I do have a little way too much experience screaming for hours over pointless things.

Say I'm hungry. My mom mentions that there's a sandwich in the fridge I could have. So I get it, and take a bite, and it has mustard on it. I hate mustard. The taste makes me want to throw up. My mom knows this, and it's like "OMFG WHY DID YOU GIVE ME A SANDWICH WITH MUSTARD YOU KNOW I HATE THIS OMG" and I'm screaming that and then part of me is like "Calm down, it's just mustard. She probably forgot it was on there. This isn't worth screaming and getting in trouble."

But the other part of my doesn't care because there's fucking mustard on this stupid sandwich and that is completely unacceptable. And the other part of me is like "Not worth it. This isn't worth screaming. Do you want to get in trouble?" And sometimes (more and more lately) I realize that it really isn't worth it and go to calm down. But other times I can't stop. I know I should but that fucking mustard was on my sandwich and it's not fucking fair do you not understand I hate mustard? This is exactly like the other time when ______! And then I'll start screaming about that.

And you say, "Well, if you can calm down sometimes then why can't you calm down all the time?" and my answer is "I don't know." It takes a ton of effort to calm myself down, and if I'm tired or if I've had a bad day sometimes I literally can't stop. I don't know why. I'll be thinking "Stop. Stop. This isn't worth it!" but at the same time I'll be yelling over mustard or whatever. It's like I want to stop but don't know how. When I do manage to calm myself down, it pretty much has to be within the first minute or else I'm too upset to stop. I can hate myself all I want later, but nothing matters in that moment except for the mustard.