Those who call discipline an edge never understand what an edge is. A losing trader can be more disciplined than a winning trader. He just needs to stick his losing plan strictly. Then he is perfectly disciplined.
Well, you need more than just a few tests of validation to become successful although that might be a start. Moreover, instead of relying on personality, you need a set of rules to fall back on. Trading should be a rule-based system. Whether you're discretionary or algo, rules should be rigidly followed.
The market doesn't care how hard a trader is working...to solve that magical puzzle of generating money from the market. There are a ton of old fools in this business with decades of so-called experience, but have nothing to show for it. Intelligence is more important than some guy who is beating his head against a wall getting nowhere, while doing the same thing over and over again. 99.2% of people in this trading game are failures, that's a given. What makes the .8% winners....ask yourself this,
25 Rules of Trading Discipline https://www.elitetrader.com/et/threads/25-rules-of-trading-discipline.128027/
I think I The honest truth is that success is deterministic. You cant really learn it however hard you try. Some are lucky for short period of times, but successful armchair traders had a long career as pros making money for other people. So a back ground on Wall Street or at a successful hedge fund is very important in my view. You get the odd exception but they are statistically a fart in a storm: small impact. I learned discipline by being "disciplined" for 8 years by pros.