Currently I own a lot of altcoin, I bought them at ico. My account is being downgraded several times. There are many people who say bitcoin will continue to decrease, so altcoin prices also decreased compared to USDT. So now, should I sell them and wait for the price to buy back? Give me advice 
Look, there is nobody who can help you and give the perfect trading advice, so sell. Really, sell. If your coins will rise you will get the most expensive experience in your life and will never do it again. Or you will gain profit if all continue dipping. So sell