Have your ingredients ready BEFORE you start cooking
I’m notorious for this (and still am). I’ll chop up that onion I need to start a recipe with, toss it in the pot with oil and figure I’ll have the rest chopped and ready by the time the onion is cooked. Suddenly I’ll be 1/3 into the work and my onion is done. Thus I end up adding the rest in a panic.
Prep work is called “prep work” for a reason. It’s when you take the time to prepare your ingredients before you start mixing and/or cooking them. You’ll notice on any cooking show the chef will have all the ingredients laid out in small dishes, so they can relax and concentrate on the moments they mix and apply heat (if needed).
Now some could say this is “worrying too much”, but many dishes also are built on timing as well as ingredients in select amounts. You don’t want it to be the time you have to add wine to a hot skillet, and now you’re wasting five minutes digging out a corkscrew and opening the bottle while your food is being overcooked. Better to have opened the wine beforehand and measured out the amount you needed.
Still, sometimes you mess up, and thus you need to prep an ingredient you didn’t prep before. Don’t panic, just turn off the heat, get the ingredient ready, add it in, turn the heat back on, and carry on.