How to take a punch?

I've been sparring for 6 months. Our gym doesnt have many members so my sparring partners vary in weight a lot. Im feeling extremly discouraged because my form is good, I feel relaxed and keep my hands up but anytime someone bigger than me lands a hard shot on my guard or my stomach im just rocked. Yesterday i had a spar with a guy who was 20-30kg heavier than me and more advanced. Every shot he landed was making me almost quit.

Why is my body this fragile? What can I do improve?