Start pinpointing areas of your game that you aren't strong in. How's your last hitting and total cs? Are you ever late to early game jungle fights? Are you relying TOO MUCH on your team to be there for you (I have this problem right now)? Are you pushing your advantages? Are you taking all possible risk-free gold income, such as raptors? Are you following bad calls into dragon/barons? Are you synergizing with your team during champion select or just always using the same champion?
I'm not asking you to tell me these, but ask yourself. Watch replays of your games or think about these questions while you play. People really struggle with pushing their advantages. If you have decent health after winning a fight, do something with the advantage of missing people, don't just go back and buy. Doesn't matter how much gold it is.