How is discipline a limited resource?
If you can't do what you say you are going to do, you aren't going to succeed at any endevour.
Can you bench press 300 lb? If not, why not? Don't you have a bottomless supply of discipline? Just will it into existence.
Discipline is definitely a limited resource as everything in the world is. Not discipline itself, but the quantity that you can put forth in a short period of time. Yes it will replenish with rest, but how much does it take to do a job and is that a reasonable amount? If I try to hold my breath until I die, I won't be able to do it. At some point my body will decide it wants to live and will override any amount of self-discipline that I have decided to put forth. If I try to saw through my own hand, I might break the skin but then my body chemistry will change and suddenly the discipline that I had decided to muster is gone.
Traders deal with fight or flight responses. Those types of chemical changes in your body can do funny things to your brain. The better pursuit is not to increase discipline, but to decrease the occurrence of chemical changes in your body so that the discipline you do have is more than enough to do the job.