I'm getting worse at the game. Why?

Whenever I pick champions that I feel confident it, I get rekt in lane or I consistently get ganked by the entire enemy team. One only has to look at my match history to realise that I am quite bad. How can I get out of this "tilt". I know how to play, but I just seem unable to apply the theory into my gameplay. I am quite knowledgeable of the game, I just suck at playing it. My friend thinks it's because of the time I spend on the boards, and I'm picking up a lot of different playstyles that aren't my own. I really need help, because it's really starting to annoy me.
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