the first thing i'd work on is my cardio. if your cardio sucks (i'm not saying yours does it might be really good) its gonna be hard to focus on technique in class.
now to work on your pain tolerance i'd suggest getting the hardest heavy bag you can find. throw some punches and kicks without wearing gloves or any other padding. after a while your knuckles and shins should get stronger. another good way to strenghten your knuckles is doing pushups on your knuckles. when this gets easy try doing them on cinder blocks.
"PAIN without love,
PAIN can't get enough,
PAIN I like it rough 'cause I'd
rather feel pain than nothing at
- 3 Days Grace