Mental self-control is something that is trained. You don't have it at the beginning. It is linked to the fact that one develops an analysis and trading methodology. The repetition of a way of doing things gives confidence and security. When you develop a systematic process, you can have a method of assessing the probability of success or failure in each operation, and over time this allows you to define well the amount you want to invest in each strategy. After months/years you will have a book of operations (track-record) on which to work, analyzing successes and errors, and from there you will be able to locate if you always make the same mistakes and how to try to avoid them. You will also be able to focus on your successes and encourage them to occur more frequently.