Hindsight is a bitch alright. I had 300 ETH bought in the ICO, I sold all around $3 each and I thought I was a genius with all that 600% profit. And then years later on even with this Ethereum downtrend, I wish I waited for much longer.
Sorry to read about your investment mistake. We all make them, we cannot win them all. Sometimes we sell early and sometimes we hold on to them for too long.
Do you remember how much you paid in the ICO for the 300 ETH?