Buying periodically doesn't mean you buy on every crash and every day. You keep buying monthly or quarterly when you have money to invest irrespective of the current price. The FOMO comes when you want to buy on every crash.

Also you agreed that you never know how long the crash is gonna go for. What if it stops the moment you sold and start going up? You never know. You can never predict the market. Trying time the market is the worst thing one can do.