'Even though I walk through the valley of the shadow of death, I will fear no evil.' Why does the psalmist say he is unafraid?

The comfort in Psalm 23 is not the absence of the valley but presence within it. The shift from talking about God ('He') to talking to God ('You are with me') happens precisely at the darkest point of the psalm.

Psalm 23:4 · BSB

“Even though I walk through the valley of the shadow of death, I will fear no evil, for You are with me; Your rod and Your staff, they comfort me.”