How we can build self-confidence?

Self-confidence rests upon accomplishment.  And self-confidence can be best built upon accomplishment that follows failure, in my opinion.  When we fail, we learn from our failures.  I believe it was Thomas Edison who said he learned 99 ways to not make a light bulb in order to make one that worked.  And in fact, when things come too easily, it creates a kind of false self-confidence. We gain the strongest confidence when we have to grapple with something, not when the road to success is smooth.  You will gain confidence by challenging yourself, no matter how hard the work and no matter what the length of time. We all have some areas of endeavor that come easily to us and others that are quite difficult.  What happens to us is that we are naturally drawn to the areas that are easy, and we tend to avoid the more difficult ones.  The consequence of this for those who never challenge themselves is that sooner or later, they encounter something they cannot do, and they lack the tools necessary to learn how to do it.  This is what I think of as false self-confidence. These are people who do not know how to persevere.  If I had had my way, I would have taken nothing but English courses, in high school and in college.  But no! Someone made me take chemistry and biology and so on, and I had to take courses in many different disciplines in college.  Going outside of my comfort zone gave me true self-confidence, struggling with the table of elements in chemistry and the big "B's" and little "b's" in biology, among other things. It is perseverance and hard work that bestow the most confidence, and challenging oneself, through failure and beyond.  It is Miss Frizzle from the Magic School Bus who perhaps sums this up best, as "Take chances! Get messy! Make mistakes!" I have included a link to an article on self-confidence, too, for some more general advice on the matter. 
https://www.psychologytoday.com/us/blog/hide-and-seek/201205/building-confidence-and-self-esteem