Selling too early/Selling too late. Some people do cry about their holdings being dumped to oblivion blaming the coin for their losses where they can avoid it themselves. Learning the feel of the markets is a must and atleast a bit of knowledge on how charts works. Tho if your buying btc, selling too early must be the common encountered problem since bitcoin time and time again proved that it reaches new heights everytime.

IMHO, you can never go wrong with bitcoin. (For long term atleast) given that it has proven, time and time again that it increases in value on a yearly basis. Every year its annual low is higher than the previous one.

If you monitor the prices you could really tell when you should buy/sell. Yesterday, i did manage to sell my XRP while it was at .69 before it dropped to oblivion. Check your support and resistance levels as this may help your trading.

With just the sheer amount of alts and listings per month, Binance, Huobi and Kucoin are my picks. You can just throw in your money there and experiment with alts as they have huge libraries of alts on their spot markets. You also cant go wrong with those three in terms of big marketcap coins aswell.

Binance is the consensus #1, Huobi is also decent, Kucoin is like a cheaper version of binance and i don't mean it in a bad way. They sometimes have cheaper fees compared to binance and has offerings that are similar but easier to participate in.

Among those three, kucoin is my most used exchange tho not by a large margin. My go to markets are with ETH