Put it simply, amateur traders are always grumbling about their trades.

If they pick the right stock, they will regret not buying enough.

If stock price rises after they sell it, they will regret selling too early.

If stock price drops after they sell it, they will be happy for making the right decision, but if the price subsequently rebounds to higher than their selling price, they will grumble.

When they see stock price escalates or plunges and they did not take any position, they will grumble for not taking any position.

I feel that it is pointless focusing on what has already happened. The most important thing to do is to learn from "mistake" and move on. The market is always there. Another opportunity will emerge in the future. We just have to be alert at all times.