Anchoring

The tendency for an initial number to influence a subsequent judgement, even when it is irrelevant.

In investing the commonest anchor is the purchase price, which determines nothing about the future and dominates decisions about whether to sell. The second commonest is the fifty-two-week high, which is a fact about the past presented as a reference point.

The defence is to restate the decision from scratch. If you did not own this, would you buy it today at this price?

Price paid is a fact about your history, not about the company.