The market is, by its nature, a most merciless judge. Every day, the efficiency of your judgment goes on trial, and when the gavel comes down with the closing bell, your sentence is automatically reflected on your ledger sheets. If your judgment is good, then you’ll consistently make money. But if your judgment is bad, then your survival depends on lady luck who, at best, will usually serve you for only a little while.
Being under such relentless and continual pressure can be an enormous psychological and emotional burden. The constant stress and tension of trading can take a tremendous toll on you, both physically and emotionally. But only if you let it. To avoid paying the price of mental and/or physical exhaustion requires an exceptional level of integrity in thought, emotions, and actions.
– Victor Sperandeo, Trader Vic II: Principles of Professional Speculation