Once you have paid for something there are some real advantages to forgetting that cost.
As an example, you buy a house for $200,000 and a few years later you need to move to a different location. If the value of the house has dropped to $150,000 should you not sell just because it’s below your purchase price? What if the alternative is to keep the house and lose even more money on the mortgage, insurance, taxes etc.?
While losing money on a transaction sucks, losing even more money because of ego sucks even more.
Once the money has been paid, you can’t take it back so it makes sense to move forward rather than keep looking back.
Have a great day!