In yr1 my eldest was very sensitive, and displayed some worrying (or so I thought) traits-he was scared of hills/stairs, wouldn't join in in PE, and school plays were excruciating to watch, as he stood terrified, copying the teacher's every action. At one point, as his reading seemed such hard work aswell, I voiced concern about dyslexia/dyspraxia.
Fast forward to just finishing yr3 and he's a different child in many ways. He does cross-country & football, runs everywhere instead of walking, and today he's the narrator in the school play.
I'm not trying to minimise, and I know there's nothing worse than people saying it will sort itself out, but I honestly believe that if the school is a happy, nurturing and supportive environment, as the child moves through school confidence improves greatly. They're going through so many new experiences, I think over time they get to understand that it's not always the end of the world, even if something doesn't go well, and they develop their own coping strategies (with your support).
I feel for you, it's a real worry. You just want to scoop them up and protect them forever!