You don't say how long it has been since your surgery. The pain will diminish with time. It took about four to six weeks for my pain to diminish, and now more than three months later, I still have some pain. You should talk to your doctor about the pain so she can examine you to see if you are healing properly. If you are, there is nothing much you can do but wait it out. Try to keep busy and do things you enjoy to take your mind off the pain.
My doctor prescribed pain meds for me, but I thought it was best to take them for as short a time as possible. After the first few days, I only took one pill a day, at night, to help me sleep. You could discuss this with your doctor. I wanted to stop the prescription meds as soon as possible so that I could go back to driving.
Are you wearing a mastectomy bra? Maybe it is just me, but I feel that the pressure of the foam inserts against my chest helps with the pain a little.
I wish I could be of more help. I wish I had something more to say besides grin and bear it and talk to your doctor. A mastectomy is major surgery and pain is to be expected.