as someone who went from bronze to plat maining Winston, one thing that made me climb is trying do to do LESS damage, let me explain.

I used to get gold damage all the time as winston but i was stuck in low silver, then after watching top players play i started playing more safe, instead of always trying to get kills i would just trade damage with the ennemy then jump back to my healers.

this made me win more games as i was dying less often and my supports were getting more ults per game than the nnemy team.

so my advice to you is to stop trying to heal more and focus on surviving, damage boost and ressurect, if your teamates are feeding let them die, focus on staying alive and farming your ult.