My number 1 rule is "never average down a losing trade".
Vijay Bhaskar
When market is moving against you, it is telling that you may have entered a wrong trade. Planning a trade include setting a stop loss and exiting the trade when stop is hit.
Treating every purchase a separate entity means living in denial and giving consolation to your heart when deep down inside you are aware that your overall position is in loss and you will have to book that loss sooner or later.
Successful traders do add to their winning positions. That is the only way bigger and successful traders make money. You think bigger investors like FIs and mutual funds invest in a stock only in one go?