Watching a Down Arrow

Like many people with any money in the stock market, I've been watching my investments go into freefall mode over the last couple of weeks. I am comfortable with losses since all the money I have invested is in retirement accounts, and therefore extremely long term (40+ years) but when you start seeing numbers like -11%, even with that long term outlook it takes some deep breathing and repeating of the "i don't need this money for another 40 years" mantra to not hit that sell button. What goes down must come back up eventually, right?