That is not the mistake you have made, there will be many people who have experienced the same thing with you even they are an old investor in cryptocurrency. Every place of investment will not be predictable easily and correctly, everyone who predicts will find difficulties and they do not know the price that will really be achieved. And also, every investment place will provide benefits if someone chooses a long-term investment, this happens at ETH, you should not think about selling the ETH you have in a loss condition, you just have to hold until market recover.