Pick one single role. Branch out only a little bit. I put myself from B5 to G5 playing nothing but Sivir or Jinx. I spread out and occasionally play as a jungler or support if no one else on the team is willing to. Put me anywhere other than Jungle or Bot Lane? I will feed so hard that even Ethiopia might have a chance at moving out of third-world status.
I started out as a jungler because it taught me everything everyone has stated above me, I cannot reinstate the importance of map awareness, especially in Bronze, it beats out almost every other mechanic in the game. Got a good CS and KD/R? Great! Look out for that Lee and Vayne bearing down on you. Oh look, there's the nexus.
Karthus, Fiora, and Corki are all rarely played for a reason: They just don't match up with what a team needs in this meta. Karthus is cute, but he needs too much gold to get ahead, and midlane sees far too many assassins that can burst Karthus and get out before his Wall can take effect, which is brutal. Fiora brings nothing to a teamfight, except randomized burst. She can't focus properly if there's 3 people together, while yes, she was voted most likely to get a penta after snowballing, its a team game. Get some hard CC as a top-laner or a jungle. Corki... Just lol. Lucian, Vayne, Jinx, and heck, even Ashe walk circles around him, he just can't put out the damage he used to with the same items as almost every other ADC. The meta exists for a reason, and you gotta be Faker level to break it. Which, lets be honest. That's one in a million, and regular Joes like you or me aren't it.
The second most vitally important thing is to never be negative. If you're doing poorly, don't blame anyone else. You did poorly. Own it. You can't learn if you don't realize it. If someone else starts flipping shit on you? Ignore them, don't fan any flames, they just want to be salty, let them be mad at a wall. Communication goes under this. If you don't have anything nice to say, just don't say it at all.
That's just what I've noticed, at least.