I have had many mammograms and I am glad I did, because an early tumour ( invisible and unfeelable) was found and removed. I have found that MOST mammogram operators are wonderful. They get you to stand at the right angle to the machine, manoeuvre your boob into position gently, lower the top plate far enough to flatten without crushing, retire behind their glass screen and press the button.
However I have had one or two who do seem unaware of just how far down to lower the plate. It sounds as though a few posters have drawn one of those. If yours is one, say clearly that it is painful.