Buy more then 1 umbrella because redundancy is useful in its self. If you are really talking the longest bear markets I ever seen, that'd be the decades long depressed gold prices which is just fine if you are using it to store solid value for retirement that cant get wrecked by the next bank crisis. Buy a gold coin (whichever your nation may have had historically as currency is often best) for every working year of your life, so the idea is to distribute not be a speculator so much but obviously as you near retirement the idea of solid value becomes a greater idea then all the gambles people take in their 20's
So far as Bitcoin is concerned, everything happens in a micro kind of scale to it. We consider a year even to be like the decade in gold market might be, that figures as the natural cycle to crypto is far quicker where as a gold mine takes a decade to establish to naturally it moves far slower. Same kind of deal could apply, consider time distributed accumulation and really only a small amount taken from spare funds. If its not spare you may feel pressure via pricing which means you are holding too much probably.
Also another take from 5 years in crypto I would point out BTC is not leveraged naturally, the supply never alters no matter the price which means its very constricted and can spike due to that. Eventually demand is served by smaller divisions made in BTC and a higher price overall, dont sell out completely. Save your change in a virtual change jar and consider that a hundred year flood type saving (but in BTC timescale of course)
A proper market gains something even in the declines, its ongoing. Dont be negative over 1 number when the world overall is very positive