Got the same problem, have tried to main a champion or two for each role. But I end up playing 2-3 matches with that champion and just have to play something else. Can't stand it! Did suck it up and played a bunch of Darius top and climbed to Gold 2 though. But after about 30 matches with him I can't stand it anymore.
Started to enjoy ARAM a lot as well, you never know what you get, and it is great for testing out new stuff. I wish they had a mode for Summoners Rift, with the same champion system as ARAM. Could even be vs bots and I would be happy. Loved the URF mode when we got a random champion, sadly I was busy that weekend and only got to do a couple of matches.
I really like testing new stuff out. But guess I need to suck it up for the last part of this season and see if I can climb to Plat for the first time, I do believe I got what it takes if I focus on a couple of champions.
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Never seen one of those coaching movies before, there was a lot of helpful stuff there! So many small details I wouldn't have thought of to be honest. And I guess thats why we're "low" elo, while people like him are high.