Fear not, for I am with you. This is both a command and a promise. Fear, worry, and anxiety are often sin. When the God who rules over the nations as described in Isaiah 41:2-4, the God who chose us and loves us as described in Isaiah 41:8-9, when that God tells us fear not, we must take it seriously!
We are much more prone to fear and discouragement when we are alone. But we are never alone because God has declared, fear not, for I am with you.