Mastery 7 doesn't mean anything. It really doesn't. Anyone can get lucky and get a good score on a champion that they play fairly frequently a few times.
What really matters is consistency. I am mastery 7 on Nidalee, Teemo, and Azir. But, on all 3 of those champions I am very feast of famine. I am either the hyper-carry or I am completely feeding out of my ass. I tend to feed out of my ass more often than I hypercarry.
Scale back your roles and champions that you play. Start out with 2 roles and maybe 4 champions. If you can play all of those very consistently and you start to climb then introduce more champions into your pool. Stop introducing champions into your pool when your performance starts to suffer on the champions.
This game isn't hard
If the game wasn't hard, then you wouldn't be struggling to climb.
The bottom line is this: If you only have played a champion for 40 games and you are facing someone who has played their champion for 200 games, then you'll almost definitely lose. They will know the matchups, powerspikes, buildpaths, and how to pull of combos better than you do. They have the sheer experience from more games.
You cannot learn how to play specific matchups if you've only played a dozen or more games on one champion.