Shaking hands after a brutal loss with your head up is a skill most champions learned before they ever won.

Composure after a loss is a separate skill from winning, and it tends to be learned earlier. Being decent while it still stings means acting from something other than mood, which is the same capacity that carries you through most other things. People notice it, though that is not really the point. It steadies you first.

Next time something goes badly, finish the small rituals properly: the handshake, the thank you, the tidying up. Feel the disappointment afterward, fully and privately. Keeping those two apart is most of what people mean by holding your head up.