Well I will talk on the aspect of invest what you can afford to loose , I think this doesn't even hold only in investment but aswell as trading And that's not the purpose of this post though 
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However, the best practice is to invest what you can afford to lose right?? In the case of Btc, its an apex coin so you can expect it to crash drastically over night even if there's a crash , there's always a little profit to settle with comparing to your entry and the amount invested, but it's good to just think it that way besides investment needs follow up so you can always determine the point opt out of investment.
Mate, there is no need to invest what you can afford to lose on something because there is a probability that the investment will be a failure one day, even though you invest with the money you can afford to lose and the investment doesn't yield out anything, you will be angry because you have already hope of making extra money from the investment. Also, it is best to invest with money you will not need for the long term so that if the investment does not yield profit quickly it will not affect your daily needs, and you can depend on other things that give your money to run your life. . However, the best practice is to invest what you can afford to lose right?? In the case of Btc, its an apex coin so you can expect it to crash drastically over night even if there's a crash , there's always a little profit to settle with comparing to your entry and the amount invested, but it's good to just think it that way besides investment needs follow up so you can always determine the point opt out of investment.
I believe this notion of investing what one can afford to lose come from the fear that the price can drop so sharply; if this happens, how is it supposed to affect a long term holder when you know the price will always turn after the moment of down trend. Maybe most of those proposing investment of what one can lose are suggesting that the price of Bitcoin will get to zero someday or that Bitcoin might be banned globally thereby making it worthless. These are baseless conjectures in my opinion.