In short: self-esteem comes from focusing on and defining yourself according to your (spiritual) potential and not your accomplishments. Your accomplishments are by definition limited so you will alway feel short and always feel bad about yourself if that is how you define yourself.
Your spiritual potential is unlimited so you will feel good about yourself if that is how you define yourself. If we would only realize how much we accomplish with every word, thought, or action! According to the Nefesh HaChaim, there are many “worlds” besides this one, and with every thought, word, or action, we make tremendous changes in all of these worlds. If you ever said one nice word to someone (even “hello”), you have done something so amazing that you can’t even imagine!!!!!