How do you avoid learning bad habits early?

Most bad trading habits form in the first months: moving stops, averaging into losers, sizing by confidence instead of risk. Unlearning them takes far longer than learning them did.

What would you teach a beginner to prevent the habits you had to break?

• the habit, and how it formed
• what broke it
• the one rule you wish someone had given you on day one

Experienced members, one habit per reply keeps this thread practical.