Maining a hero isn't enough. You need to be aware of your surroundings, help the team win by picking the ones who are doing the most for the enemies, healers should be prio 1. If you suck at aiming, maining a dps isn't enough, learn to aim. Practice the shit out of quickplay. Doesn't matter if you win or lose, you still get practice. Also, watching streamers and Youtube guides has helped me a lot. Even watching a streamer in your own rank can be good, notice the common mistakes and learn from them aswell. Group up with players you have fun with and make call outs (who's low, who's on high ground etc)..