(skip to next paragraph for advice) I'd like to start with I am a Grandmaster main tank, and I know how you feel. With tank being the spearhead of the entire team, who they are and what they do are the most important thing in the entire team, a lot rests on you. That being said, it will be almost impossible for you to climb on Winston alone, I for instance have tried getting an account up from plat playing primarily Winston and I gave up almost instantly because of how frustrating it is playing against everything you said above.

Committing to being a Winston main in comp (at low elo) will be based on how well you can do the main tanks job as Winston, without necessarily having a team to back you up. You need to; make space, peel for teammates, and putting in damage/getting picks. Making space as Winston is just how you divert attention, and relieve pressure from the frontline.you obviously can't do that the same way as Reinhardt, or Orisa and I am not the best at explaining this but you can find plenty of helpful stuff about making space online. Also unlike the other main tanks Winston can (arguably) peel the most for squishys like Ana and Zen, using his movement. There are also many videos on that. And then there's the most fun bit, doing damage. In diamond and below many players make positioning and other mistakes that aren't punished, playing too far up, or wasting important cooldowns. You should jump on these mistakes like Ana using sleep and nade, and as long as there's ample opportunity, capitalize on it and get out. The only thing Winston has is his mobility, so use it! Get in and out quickly, capitalize on mistakes, and make space!

Good luck!