Do not be anxious
How does one not be anxious? How can one decide to not have that knot in one's stomach?
I think when the Lord tells us not to be anxious, He is like the mother who says to her small child, “Don't cry.” Within the comfort of the mother's arms, the child is often able to stop crying.
"Do not be anxious” is not so much a command that we must discipline ourselves to obey. It is our Lord's word of comfort and compassion to us. So, like the comforted child, we become less anxious.