I for one, don't really think it exists, but whether you do or not isn't what matters.
When I first started ranked, I was really bad. Very bad. Really, REALLY bad. I thought I was ready, but turned out I wasn't. At that time I believed in ELO Hell, that I was actually good but I was stuck with bad teams. Everyone I know who is much higher than me has constantly said I wasn't carrying my team hard enough. I just kinda tossed that statement aside and kept trying, but I kept failing.
So I stopped doing Ranked. I went back to playing regular PvP matches. Slowly but surely, I bettered myself, and found that there is one particular lane I was best at, and that was Mid lane. I could play almost any champ and do very well.
But that wasn't enough, I wanted to pick ONE champ and try to be the best I can be. I ended up getting a decent champ in all roles, just incase I didn't end up mid in ranked, with enough practice.
So I do very well with Brand, so I decided to do a Ranked game with him. I won! I didn't do as well as I hoped, but it was a win nonetheless, and I kept my Kills higher than Deaths. Slowly but surely, I have been using champs I'm best at on a regular basis, and then using them in Ranked. What used to be a 3-11 win/loss score is now 11-13. Its still below, but I can get higher.
So practice hard, think positive, and don't give up. Play your best champs, never try out champs you don't know how to play, and don't practice champs that usually get banned. I suggest trying to get great with a carry role, but if you're better in another, props to you and I wish you the best of luck.
Those are my 2 cents.