The problems attributed to lack of discipline in trading rarely are due to a lack of discipline. In the same fashion, a trained trader will virtually never take an impulse or revenge trade. It just isn't part of the trained traders "vocabulary".
If you are trying to be professional, you shouldn't make it part of your vocabulary. You make 1 revenge trade then you should ground yourself from trading for 5 days, the next time 10 days, either you will stop revenge trading or stop trading.
Discipline is harder to correct in this manner because we need to look at what discipline problems really are and what they really stem from.
* Trader not taking a stop loss.
Most of the time, not taking the stop loss is due to uncertainty about whether it is the right course of action to take the stop loss. So, in this case the trader's discipline problem was an "uncertainty" problem
* Trader decides to change the rules of his system or risk more then his plan called.
In most of the cases, this is not a discipline problem either. The real problem is not being clear to oneself what one really wants. One part of the trader thinks he's not willing to risk more then $x while another part says, he's really okay with risking $y. This is often a battle inside the traders mind of competing interest. The next explanation will clarify.
* Global vs Local optimimums, Maximizing Profit vs Minimizing Risk of Failure (Maximizing Survival)
This is a bit hard to explain but is likely a source of traders frustrations. Due to a traders account size and what he feels psychologically okay with losing, he may decide he needs to trade with a $500 stop loss. However, the true optimal stop loss for the trade might be $1,000. So, if we were to take $1,000 risk per trade, he'd always make more at the end of the year. Well, what might happen is the trader decides to maximize profit in this situation versus maximizing survival. He really wasn't undisciplined, he just decided in this instant to change his goals but doesn't understand that.
* Unexpected events
Discipline itself refers to acting in a way in accordance to ones rules. However, that implies that one has considered all the possible things that can happen. But what about when things happen that one didn't plan for? Is discipline the problem or a lack of planning?
As such, rules have real limitations whereas principles can be effective. I call this Principles Over Rules.
So, if most of traders problems they think are discipline related aren't then what are most traders real discipline problems? Good question and I'd suggest lack of study, lack of off market work, lack of testing and idea generation, etc. Basically, not doing the proper amount of work. All of those are discipline problems.