I've have several regrets in cryptocurrency and Infact the regrets happens daily, either when I sell before a price pump or hold and see my portfolio drop.
My greatest regret was not starting cryptocurrency earlier than I did, though I started as soon as I was introduced and had to make so many mistakes along the line due to my ignorance and the person who introduced me was also a novice and didn't know much.
I regret not buying bitcoin when it was less than 500$ and rather wasted my funds on hyip and ponzi scheme.
I regret not buying eth when it was 12$, though I didn't know how to buy it at that time and was busy with my academics.

But in all these regrets, am glad of my experience so far and believe that my mistakes were necessary to bring me to where I am now.