My experience was regretting, when I sold all my Eth in 2015 without knowing that it could change my financial ability in 2017-2018 Bullrun, that was attained the All-time high price of Ethereum, While the second regrettable mistake in crypto is during 2018-2019 when I'm "holding" many altcoins, in hoping to gain some profit out of it, but it was ended up nothing but back to back loses due to the bearish market situation at that time.