It seems like there's a choice being given here: be strong - or don't. The understood subject is "you" - you be strong. That's just the first part of the psalmist's instructions. He goes on to say let your heart take courage. This implies another choice. We can let our heart take courage or reject it.
Today we must trust that He is giving us His strength to walk this journey; and He will encourage our hearts - if we let Him. Think about allowing His strength and courage to take over the space in your heart today.Allow His peace to walk you through whatever you may face today.