If people know when price will back down and up, maybe a lot of people will be rich and no one complain or regret why not sell or why not buy bitcoin when it is on right price. But in other side, it prove that bitcoin is really volatile and it's price movement is unpredictable.
That's true, unfortunately no one knows, the market is very unpredictable that's why it's considered a high risk investment.
Regrets are all over in the community, people would want to see at ATH but we are just human, we don't control the price, and it's our fault as we got greedy that time.
One thing in crypto is that never regret because nothing is in your hand else you would be too much wealthy. Also never become greedy which can cause the major losses so buy/sell accordingly which will help you make money and will not miss the opportunity.