The biggest mistake anyone can do is still holding unto their altcoins when bitcoin touched 60k weeks ago and still holding strong when the fall began weeks ago, imagine someone still holding HEX token when the coin did 40x already? If you are someone like this friend I don't know what your aim is but sorry to say you are doing it wrong. Check yourself cos it's possible you are struggling with greed.
I don’t have some BTCs right now, but most of my holdings are Ethereum. Even if ETH crashed from it’s previous all-time high to somewhere around $2,000’s, I am still at a good profit and now it’s hovering around $3,300 to $3,400. I’ve bought ETH sometime on early 2020’s when it was still below $200, so I don’t have any problem holding this for long and I am still betting on it to reach a new all-time high before 2021 ends.
It's not really a mistake, it's a personal strategy. ETH is the perfect choice and you have chosen the right time to buy, ETH is still undervalued compared to its value. I also believe that ETH will reach a new ATH when the ETH2.0 upgrade will launch next year. I won't sell my ETH until it reaches 10k$.