Why Do I Suck at League?

I couldn't post this in General Discussion because the Boards think I'm below level 30, so miscellaneous seems like a good substitute. When I talk about sucking at league, I mean badly sucking. I have over 700 teambuilder games and my most recent scores were 6/14, 0/6, and 3/9 and I've done much worse. I have played almost every champion in the game, whether on free week or bought, and have had very little success with any of them. I used to do well with wukong, but I hit a brick wall where I lost every single game I played. I have mastered, or attempted to master quite a few champions, but I never do well, consistently or otherwise. I have carried one game out of my 700+ games, and that was as wukong, maybe a year ago. I don't know what to do anymore. It's frustrating when everyone tells me 'you'll get better as you play the game' and I'm still doing consistently bad. Does anyone have any advice besides farming better and knowing how to harass/play defense?