Mindset and Perception

I have a question about the episode #051: Ending Family Enmeshment with The Holistic Psychologist and the clip Mental Muscle Training. I'm interested in learning about the process by which people can change their default automatic thoughts, as discussed in the episode Do this to become Navy SEAL mentally tough | Mark Divine and the clip Mindfulness Mastery. From what I understand, the process of change involves paying attention to your thoughts, noticing when they aren't aligned with the future you want, and choosing to challenge and replace those thoughts. For example, if you have the default thought "I'm not smart enough" and want to become a more confident person, you would acknowledge the thought, challenge it by saying "No, that's a lie," and replace it with "Actually, I'm very smart, and I choose to believe I'm smart." Is that enough over time to weaken the connections of the original thought and establish the new thought as the default? How does that happen? Is it through long-term repetition, or by adding another thought immediately after the one you wish to erase or replace?