Sometimes my girlfriend says I'm being too rough with her breasts, other times she begs me to pinch her nipples almost as hard as I can. What's the story?

There’s a big difference between the nipples and the breasts. The nipples are tough little things which can take quite a bit of intensity in the form of pinching, squeezing, nibbling, etc. The breasts on the other hand are very sensitive and certain types of fondling can be very uncomfortable or painful. Also, every breast is different and generally differs in sensitivity with the time of the month and the level of arousal. You have to get to know the breasts and nipples in question. Listen and respond to what your girlfriend is telling you.