how to establish confidence in buying and selling altcoin because when looking at the price down and immediately want to sell even loss
If you work short term, taking a 10% loss is an everyday thing. But at the moment is quite difficult to go wrong in this inflated market where money is flowing to both good and bad projects.
There are many good sources of trading education out there. Not get rich schemes, but real trading courses that will teach you at least the basics to answer your question on case by case basis.