When you're feeling flustered, or otherwise unsettled, just remember that taking time out can help you. Sometimes your mind is being irrational, notice it, live with it, and take the time to breathe. This is where mindfulness practice can come into play, meditation is a good way to calm down, it's worth taking the time for it, everyday if you can. It's also easier to pick up a habit if it's daily as well, like this writing can attest to.

The hard part is noticing it. Thankfully other people can let you know, but you should know when you aren't thinking straight. I guess this answer will be different for everybody. Feeling the urge to take the time to breathe should be enough, but also taking the time to breathe in itself will definitely let you know.