Trading strategies are arbitrary, contradictory, and only work in retrospect

Gies are arbitrary, contradictory, and only work in retrospect
There are many trading rules such as "Cut your losses, but let your profits run", or "Sell after any 5% retreat", "Always use limit orders", etc. They all sound like good strategies, and they all are good strategies in one situation or another. Unfortunately, you don't know ahead of time which strategy will look good in hindsight, and they are all so vague and contradictory that they have no use except in hindsight. You might just as well complain, "Oh why didn't I buy low and sell high like I knew I should?" because most trading rules are of little more use than that one. They are fine for looking back, but they don't help going forward.

After a bad trade, you can always go back and see where you went wrong, discovering how some particular strategy would have saved you, but the same strategy that would have saved you this time would kill you another time, so what use is it?