In order to achieve discipline with trading, you need not to be fearful from losing your trades due to some inevitable mistakes. You won't be losing the whole time, because you will definitely learn from your mistakes and create positive trades after. With that, discipline already develops.
And with trading comes with battling your emotions most of the time, until you learn that emotions turn out to be your best enemy in trading. With that, you want to be responsible by controlling your emotions and put limitations to it. With that, discipline also develops.
What you said is very true. People who analyze the market themselves and set their own risk levels before trading will understand even if the price does not move as desired, in other words, if they experience a loss or hit a stop loss. The positive thing is that they will record it in their trading journal, noting the cause of the loss. what was overlooked that caused the position to hit the stop loss. These factors will sharpen their analysis and become valuable experience that will be very useful in future trades.
Unlike those who trade by relying on signals or recommendations to make purchases from others, they tend to just follow along, not understanding at all the reasons for profits or losses. Their mindset is only oriented toward how much profit they can potentially achieve with a certain amount of capital.