The worst mistake that I have ever made is not holding my raiblocks coins (nano after rebranding) I sold my coins very cheap
If I didn't sell them I would have been rich by now.
I also made the same mistake when I got a lot of Nanos from the faucet and sold them in mid-2017. Maybe a lot of people would not have thought that the nano would hit nearly 40 $ by then. Hoping the altcoin season is coming soon in the near future.