Okay, so I'm a PC Player and I can say that most of the tanks don't require good aim, but as a player, these characters reward you greatly if you can aim with them. For example, your Roadhog can be taken to the furthest ranks if you can maintain an average 60%+ Hook Accuracy, while also getting good at learning your Alt. Fire range.

I find mechanics decide the later ranks, while smart play decides lower ranks. That said, you're still expected to play smarter than everyone below you, so be sure to make calls and decisions you're confident are correct, while also maintaining an effort to make sure your confidence is based in actual information.

Being a Winston main, you just need to learn target prioritization, and learning to identify openings in the enemy team, and punishing it as soon as you literally can. Closing in on a positional mistake requires the fastest reaction you can muster, so if you don't feel like you're playing at that level of intensity, speed it up. Try and be doing something productive for your team at all times, be it harassing their back-line so they're running and not healing, zapping the Rein shield so he can back the fk off, dropping bubbles on the payload for your team.

So, when I'm playing against a team and I recognize I'm not doing as well as I could be, I generally have two macro-adjustments I can apply to my game-play with just a prompt of 'Okay, time to slow it down' and playing more patiently, pensive and controlled. Find some adjectives and see if you can practice applying them to how you approach the game. If you can play fast, and play slow, in two separate 'states', you'll have an easier time in adjusting what works and what is hindering you. Just make sure your 'play fast' isn't way stronger than your 'play slow'. Balance that shit.

Though to be honest, at this point--you're diamond. You gotta pull up your big boy pants and ready yourself to face equally knowledgeable opponents in the game, where your only opportunity to win is to outplay your opponent in every engagement. That's the hard part, is playing another player, rather than the matchup or composition. It's also the funnest part, so be ready to feel addicted once you grow in your outplay abilities. Read guides, most of those are just laying out the tools you can use to outplay another, with certain strategies or sequences that're confirmed strong in certain scenarios. Implementing is the easier step, so get researching.