the thing that really frustrates me is not selling my cryptocurrency in 2021! I'm really sick,
and can't think, what should I do?, right now my assets are really down to -90%
Take it as a lesson and experience for us. No one will succeed the first time. Most of us are getting through that together because the market is unpredictable, as long as you are holding the top coins and you haven't sold them that means you haven't lost anything. Try to hold firm during this period, bear market will end and bulls will return, your assets will recover soon. Don't worry and wait patiently is the key to success.
Many people got carried away from the peak of crypto back in 2021. Looking back, that was ATH but back then, everyone thought their coins could go higher. Thanks to the low urge to sell, cryptos could come to that high so he shouldn't blame himself as many others also made the same mistake. We can only learn that as a lesson and try to be better next time, slowly DCA out of your bag if the price is good.