is there a role you enjoy playing?
do you have strengths that you recognize?
is there a part of the game you're passionate about?
is there a champ you absolutely love playing?
for me, I played sup since i was level 9 I think. I tried out janna and really enjoyed her, and one time i accidentally picked sona thinking it was janna (they were both FTP that week) and enjoyed her just as much xD and everyone told me to go bot and well eventually i figured out apparantly i was picking sup roles, and figured out ways to be a good sup....
but eventually i got really frustrated because i could never "carry" games and so i found myself in a spot like you for awhile, playing every role and never settling on a champ...
one thing I believe I'm good at is "decisions" such as when to initiate a fight and when to back away, or whether to go for baron or contest dragon and when not. I certainly lack at it compared to what I could be, like for a while i never pushed to end the game when i should have xD but it's something that I always am paying attention to, so it's something I'm naturally gonna get better at faster than other aspects of the game. so this would tell me to play champs that initiate, or champs that disengage... champs with cc over damage.
for a while I tried to main ADC but the thing is, I am so frustrated by constantly having to watch my position. if i misclick I don't do damage or walk right up to an enemy tank or assassin and squish I'm dead. lol...
so when it's down to it, my champs that are good candidates to main are sup champs and/or tank champs... I'm still very unfocused tho. I usually have a list of like ten champs that I'm "maining" which well... that's not "maining" is it xD