Have patience and never sell lower than you bought except if you are trading BTC/altcoin and you need BTC for a trade that you are sure that will give you profit.
Also, you don't have to think for hours if you should buy or sell when you are sure about something. It will just make you nervous. Most of the times that a patient and good trader lose money are after a strong crash which is normal.
You should know that you can't always win and you should never trade/invest more than you can afford or are willing to lose.