Actually people's learn from mistake. Those mistake I did began, I am realizing now. That's why I want to share it. Perhaps everyone will not agree with me. Every traders have few favorite coins. It's normal fact, but don't love too much any coin it's my logic.
I don't want to mention but I have lost many btc just one coin. Just think it will go more up and didn't sold. In a result price dropped 4X. So I want to say don't love any coin, just sale when you are in profit and go another coin or wait for drop price again and buy it. It's very difficult to recover once drop an Altcoin. One more thing, don't invest your all fund on single coin. It will loss more, who know it will up or drop.
The best piece of advice you gave is in the last line -
don't invest your all fund on single coin
I believe this is true for bitcoin or any alt-coin. Diversification is key for success.
The rest of the advice is true as well it's just not so simple to follow because knowing when to buy or sell is tricky and it's hard not to grow attached to coins you believe in and that provide you with success.