There are only two things we all need to be able to control, to be sucessful investers in anything! FEAR, AND GREED!Too many folks getting caught up looking at a 401 or other investment based retirement account as a pile of money rather than a pile of shares. The monetary value of anything comes and goes --- in the case of stocks, it pretty well always trendlines up over time. That's why the SPOT value is less important than the shares. Personally I evaluate my 401k maybe once every year for the 3 and 5 year performance of the funds. I know I'm not retiring anytime soon so it's not like drops bother me in the least. That's not to say I don't have non-investment value streams to diversify.
IIRC the worst drop in history was the Black Friday crash, and the market didn't recover there for about 20 years. But, it did recover and has grown "slightly" since then.