Had almost the exact same experience. What helped me afterward was recording myself talking through a design out loud, alone — sounds silly but it's a very different skill from just knowing the material.
Interview Experiences
Failed a system design round — here’s what I’d do differently
Froze a bit when asked to design something end-to-end on a whiteboard with no warm-up. In hindsight I should’ve spent more time practicing thinking out loud, not just knowing the concepts. If anyone has a good way they practiced this, would love to hear it.