sensitive nipples

Emilee
So we lost the shield... tried all day to feed without one and it is painful!!!
I know I have very sensitive nipples and that could be a reason, along with my son isn't very big and he can't easily fit my whole nipple in his mouth yet. 
Is there a way to toughen them up quickly?
It feels like he's twisting my nipple during feeding and it's not making it easy for me... Caved in and gave him a bottle to give them a break before I'm deterred from even pumping...