This happens every now and then. And truth be told even the best of us sell off at a seemingly good price only for the market to turn the next minute and we have a huge pump. It happened in December we saw btc pump to $30 and didn't dump much until February. Selling off at $40k mark price would look like a loss right now.
How do you decide a peak and a bottom?
It's difficult to decide where is the peak and where is the bottom. I just try not to panic and calm down so as not to sell bitcoin. You just need to understand that the concept of a fall is normal in the exchange and it'll pump again.