The key to a good investment is to treat it like play money.
Only invest an amount that is insignificant in terms of your overall financial situation. If your investment is significant to you, a sudden downturn can be stressful and leads to negative emotional reactions that may bring on impulsive buying and/or selling.
If your investment is small enough that it's not going to significantly damage your financial situation, it's much easier to let it go when these downturns happen and wait for the markets to improve.
HODL isn't a strategy that will work forever. Eventually the bull crypto markets will dry up and we'll see a gradual downturn in value. But in the current environment, being steadfast and not selling on impulse seems to be a very profitable way of approaching downturns in the market while we wait for the next upswing.