This is a very common behavioural mistake called the disposition effect: investors tend to lock in gains too early and hold on to losing positions for too long.

The most common reasons:

Fear of losing the profit. Once a gain appears, we want to “secure” it, even if the company’s outlook hasn’t changed.
Avoiding the pain of a loss. Selling a losing stock makes the loss feel real, so we wait for it to “come back”.
Anchoring to the purchase price. We judge the stock by what we paid, not by what it is worth today.

How to reduce it:

Set rules in advance, e.g. a partial sale or a review point.
Ask: “Would I buy this stock today at this price?”
Keep a decision journal to see, after a few months, whether your sales were actually right.