The Gospel of Strength - Swami Vivekananda

Know that every thought and word that weakens you in the world is the
only evil that exists. Whatever makes man weak and fear is the only
evil that should be shunned.

The greatest sin is to think yourself weak.

I beg you to understand this one fact - no good comes out of the man
who day and night thinks he is nobody. If a man day and night thinks
he is miserable low and nothing, nothing he becomes... That is the
great fact which you ought to remember.