Simple; Practise. Ye need more practical application of champions.
You wanna become godly with a champion? Play them a lot. Learn their lane matchups and how to deal with them, yes even the bad ones. Especially the bad ones.
I'll try to keep this short(Something I am utterly terrible at), but here are a few tips for getting better in the long run:
**1: Don't become rigid. **
Learn to adapt. The game WILL change and you can't stop that. Items, Champions, heck even the map itself can at any time be changed, sometimes even by champions in the game. Don't get too comfortable, because things wont stay the same for too long.
2: Find your playstyle.
Your unique playstyle, the things you do well and the things that work with you specifically. Note that this doesn't mean "Tank" or "ADC" or anything like that.
I, for instance, work best with timing, huge game-changer abilities and CC I can protect or otherwise aid my allies with, thus I've grown into playing Sejuani, Veigar and Karma, whom all have those sorts of abilities. Notice how they are all champions in completely different roles, being Jungle(Sejuani), Mid APC(Veigar) and Support(Karma)
It's not easy to figure out, but once you've got it it becomes easier to spot what you might enjoy playing and/or might be naturally good at.
3: Adapt your items.
Your choices of items matters a huge amount for your performance. Tying in with tip 1: Don't get too comfortable with a build. Learn to adapt it based on the enemy team as well as your own. Playing an AP caster with relatively low cooldowns and there's a juggernaut/kitable champion on the enemy team? Buy Rylai's. Enemy is building MR? Void staff. They got heavy burst on long CD? Get Zhonya's.
4: Number of champions.
Now, I know it may be tempting, but if you want to really get good at the game, like, get REEEAAAALLY good, don't play too many champions. Don't limit yourself to a single champion either though. I've heard 4-8 champions is a good number of Main champions to have. Of course this doesn't mean you CAN'T play other champions, it just means you should primarily play a certain few champions you mesh well with and enjoy to play :P
Now to apologize for a REAL wall of text, have a TL;DR:
- Learn to adapt. Be like the straw and bend in the storm when a tree would break.
- Find your personal style. Nothing is more stylish than what suits you individually!
- Pick items based on the current match. Why pick grass into fire when water exists?
4: Play the right amount of champions. 20 is a pretty big crowd, but 1 is kinda lonely.
If you need any help or have any questions, feel free to throw 'em my way and I'll do my best to answer/help out! I mean I know I'm not THE best, but I think I should be good enough to at least help a little bit :P
(Also if you want to you can throw a friend request my way and we could talk over the LoL client instead of flooding the board. ONLY if you want to though.)
Good luck improving further! Hope you find success!