when you do trading, do you always think about this, "buy some, sell some, hold some."
That is not really a strategy that you can apply, you can hold some of your coins and buy or sell the rest but you cannot do both at the same time, you will just lose money thanks to the fees of the exchanges and the transactions costs, to me what you are saying sounds as if you are just mindlessly trading and that is not a good strategy if you really want to make money.
Yes that is not so easy to choose the best crypto currency for trading, and the reason is there are a lot of crypto currencies in the market. Most of them are shitcoin and investing money there means only to waste your time and money, but they cannot give you anything. Therefore it is too much important to be careful while investing your money in crypto market.