Kahneman and Tversky measured it: losing $100 hurts about twice as much as gaining $100 feels good. This is not a character flaw, it is how the human brain evaluates outcomes by default.
Its consequences in investing are almost all expensive. It explains why good investments get sold in downturns, which is the one action that turns a temporary loss into a permanent one, why losing positions are held for years waiting to "get back to even", and why many people with a forty-year horizon hold a portfolio that is far too conservative.
The defense is not to feel it less, because you cannot. It is to design the system so the emotion has no buttons to press: automatic contributions, infrequent reviews, and a rebalancing rule written down before the fall arrives.