Hardness on one side rather than the other
Hello and thanks for your question, but I'm afraid I need more information before giving any answers. Have you had breast augmentation surgery and when? How long has this been going on? Please see a Board Certified Plastic Surgeon and you can be evaluated to see what is causing this and how it can best be treated.
Why is one breast harder than the other?
There aren't many answers we can provide without further information. Are you talking about your native breast tissue or did you have implants? If you've had surgery, how long ago was the surgery performed? This could be a number of different things. Please provide more information so that we can assist you further!
More information is needed in order to answer your question. Do you have implants? When were they placed? When did it begin? I would suggest visiting a board certified plastic surgeon in your area for an evaluation of your situation.