Ms. Truly, there are no muscles in the breasts so doing "breast pump" exercises will not achieve much. You can try to exercise your pectoral muscles behind your breasts to give you a little extra support, but this will not alter the firmness of your breasts.
Medico, if your breast size is causing you much anguish and distress, not to mention physical pain, it may be (e.g. back pain) worth have breast-reduction surgery. This would be a very big decision to make, so you would need to do your research and talk to experts first.
A less dramatic approach could be to get your bras professionally fitted. Wearing the correct size bra for your breasts can help give them extra support and hopefully ease your suffering a little bit.