How is your thought process when the market goes down?

I’m 26M, and have been working and investing for 4 years. I haven’t seen a major crash like 2008 or 2020 during Covid yet. How do you deal with seeing so much of your investments go down so quickly, potentially erasing all of your gains over the past few years? Do hold onto some cash for these buying opportunities in these moments? And what if you need to withdraw money for an emergency during these conditions?