I didn't read every comment, so I'm not sure if this has been said or not, but what I did is watch tons and tons of videos of all aspects of the game Hero guides, aim guides, and just watching gm players gameplay to name a few. Learning every Hero and map is very important in this game. If you're having a hard time against a certain Hero, start playing that hero. It helps your understanding of how that Hero works and what counters it. Also, learn map specific strategies. For example, As Sigma attacking on Echienwalde's first area, I like to book it to the left side and shoot their backs while they focus my team, splitting their attention. If something goes wrong, I can easily grasp and escape. (I'm mid masters so if that sounds dumb then I'm sorry but it works). Another general thing you can do is make sure your sensitivities are good and make your reticle clearly visible. If you want higher sensitivities than 100 100, then you can decrease aim smoothing to anything below 100. 

I've run out of things to say sooooo yeah I hope this helps