Confidence is a mental state with the belief that you are competent, that you have the skills to achieve what you want to achieve, and that you have a high probability in achieving it.
Confidence can be contextual or general. Contextual confidence is confidence in certain contexts. You might be confident in some contexts but not in others. For example, you might be confident in your ability to play a certain sport but not to sing a certain song. General confidence is confidence in the contexts that matter to you. You might be confident in some contexts and recognize that you're incompetent in other contexts, but you might also recognize that the other contexts aren't important to you, so your incompetence in those other contexts don't need to negatively affect your mental state.
Building contextual confidence is about identifying weaknesses in your skills and working on them so that you can justify your competency in the skill. Arrogance is the belief that you are skilled without having the skills to justify that belief. A teacher can help you identify your weaknesses, but you can also reflect on your own abilities. Building general confidence is about determining the contexts that are important to you, and building contextual confidence for each of them.
Confidence can decrease as you recognize your limitations, such as when you your skills are not as good enough for a context, when thy're not as good as they used to be, or when they're not as good as they could be. Humility is the belief that your skills can still improve because they're not as good as they could be. You can be confident and humble by recognizing your ability to achieve what you want to achieve while still recognizing that your skills could be better.
Maintaining confidence is about exercising your skills so you can maintain and remember your competency. It's about using your competency, not just having it, so you don't lose it.
Building and maintaining confidence is one of the most important skills for long-term and short-term success. Confidence can contribute to a cycle of success. Having confidence can help you convince others of your competency, which then provides you with further opportunities to build, maintain, and display your skills and confidence.