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 
If I were you I would not sell. I will wait for a good time for the market to return and your assets will be restored. So keep waiting and keeping the Altcoins. The crypto market is getting better than in 2018 so Bitcoin will increase.