At the risk of being a smartie-pants I’d just like to say this old market moral is wrong. And maybe always was.
If a trade goes into a loss, it will hit its stop-loss. Until then the trade decision has not been invalidated, and there is simply no gain in exiting valid trades early.
As for the winners, well, the really successful traders don’t let them run, they add to them.
Completely agree on the SL and the trade not being invalidated before that. From a personal perspective, I can see exiting a trade early would make sense in the longer time frames. Keeping a pair active for a month stops me from entering other valid setups from that currency pair.
But adding to a position has never made sense to me. The second your trade is active, that there is the optimal entry point, anything besides that it’s just increasing the risk. In my humble opinion, it’s much better to enter a bigger lot size, and after that gradually decrease the risk as the trade is coming to it’s end, than the other way around.
Managing a profitable position is the key question in trading. Making the most of a good situation is also the most difficult.
Managing a trade is probably why at least 50% of traders fail.