Is discipline enough to become consistently profitable?

A trader can follow every entry rule, respect every stop-loss, and never revenge trade, yet still lose money if the strategy has negative expectancy after fees and slippage.

This makes me question whether trading communities sometimes overemphasize psychology while underemphasizing statistical validation.

Which should be addressed first: execution discipline or proving that a strategy has an edge? How would you distinguish a psychological problem from a strategy problem using journal data?