It would be easier to make suggestions if you said what you have already done.

However, assuming that the torture is purely physical and assuming you have had all the usual tortures such as slapping, pinching, clamps, bondage, piercing, hot wax, etc, you may want to try pain from other sources, such as stinging nettles, insect bites (e.g. ants, mosquitos), or to use a stiff brush to scratch them or even to scrub them.

Wearing a bra lined with rough material, such as the hard side of velcro may also work.

Or if you want more mental torture, when you are very tightly bound you can have a glass or a glass bowl containing a big hairy spider turned upside down on your breast.

have fun :-))

M