I don't think it is "knows to be true" but rather, "believes or would like to be true".

If I believe I have a sense of humor and you do not think I do and independent testing says I do not, I still have self esteem because it's what I believe about myself that trumps everything else! You are the only one in your world!!!!! The rest of us are just props on your stage, minor characters; you're the only true actor.

I ask someone I love and trust (my husband) to describe me and then go from there; I trust his judgement so I trust what he says about me. I want those good things to be true, I want to believe they are as he describes! Guess what? I act like what he describes me as; he thinks I'm competent, I step up to the bat and act competently; he thinks I have good judgment, I make good judgments!

There is a little truth to your "know to be true" but that can come after "suspect"-because-someone-else-said-so. The trick is to make friends with yourself so you realize you do not always have to be honest, brave, and true; just most of the time! And, the times you are not, you have to recognize you have not acted that way (the action is not "you") and take responsibility for yourself.

The best boost to self esteem I know is taking responsibility for yourself. You only "judge" yourself on concrete actions of your own (not thoughts or feelings or other people's thoughts, feelings, or actions) and, you are a "fair" and merciful judge. If you make a mistake; you recognize it, apologize for it, and fix it as far as possible. In short, you do the best you can and recognize that you are doing the best you can; that's all self esteem is, knowing you are doing the best you can and feeling that is good enough and you are glad you are that kind of person.