I totally understand where you are coming from, i had such severe bruising I thought I had broken a rib it was that bad. We are really not meant to take the sports bra off at all, because the boobies have to stay still and not move about (no jiggling lol) if you are in that much pain maybe sit watching TV, very still with your bra off and icepacks on. but fasten up your bra if you need to get up and walk about. Its really important to keep them in a bra so that they sit in the right position as u heal.
My nurse in hospital said as long as I am sitting very still I can undo the bra for a bit xx
good luck hun, hope your new bra feels better.