I didn't wear a bra for a long time. I think that might be making you more uncomfortable. I think I was advised not to wear a bra during rads but I wouldn't have anyway. I wore slightly small soft camis. They give some support without any irritation.
If you get a size smaller, or even two sizes smaller, than you would normally wear in a very soft fabric it sort of hugs you comfortably. It holds you in without pressing or rubbing on anything.
Good luck. It will be over soon.