Can you say "I should have stayed calm" when you're talking about your state of mind over a couple of months? Or would it be more suitable for a shorter period of time? I'm looking for an expression to describe someone's state of mind not be overwhelmed by the amount of responsibilities to lead a big event during the course of organizing and executing it. Like "If I had been more _____, I wouldn't have made so many stupid mistakes."

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You’re welcome! I just happened to open the app 😁 Anyway, I think clear-headed works a lot better. I would not say those adjectives work only for short periods of time, because you can also describe someone as “a calm person”, for instance. But in your case, I think you’re right to try to find something that sounds a bit more natural. Maybe: “If I had been more organized (and/or prepared), maybe I wouldn’t have made so many mistakes.” Alternatively, “If I was more focused, maybe I wouldn’t have made so many mistakes.” There’s so many words you can use😅 Hope this helps!