Many people here want you not to "force yourself" to have a main - I strongly disagree.
The issue is that this game is very complex - and the higher the level of play, the higher the complexity. If you try to play everything a little you will simply get overwhelmed with the challenges of the different roles and champion. In my experience you should really focus on a role/champion as this allows you to understand the micro/basic challenges of what you play so that you can actually focus on the macro game. I am sure you too realized that whenever you try a new champion your decisionmaking seems to suffer because you are too busy with mechanics/simple questions of what to do.
But it seems that you realized that already and that people simply ignored your actual question.
So how do you find your main champ/lane?
I'd first approach the choice of lane as you will probably not suddenly be an OTP so it's better to decide on a lane rather than fixating on a champion.
Top: 1v1 focused, wave manipulator, choice between tank or splitpusher, somewhat isolated from the flow of the game
Jgl: early game focused, decides the game flow, plays god, has to read situations quickly and act accordingly, needs to be confident
Mid: most important lane, 1v1 if you want, very different playstyle depending on champion, your roams decide games and you are literally in the middle of everything
Adc: love depending on your support?
Sup: love depending on your Adc?
If you really can't decide then just throw a coin. If you do not have a premade I'd generally suggest staying away from botlane as the other positions are more enjoyable solo imho.
And then focus on your champion. As you don't seem to have a clear preference try something with rather complex mechanics as this will allow you to engage deeply with the champion and their mechanics. I for one main Riven and I spent a lot of time on learning mechanics and understanding how they work (buffering, animation cancelling, resets etc.).
Make a list of complex champions for the lane of your choice and if you can't decide just choose the one that looks the best to you. Then look for tutorials and guides on how to play that champion and spend a lot of time in practice mode. Take a look for OTP streamers and follow them, this will help your attachment to the champion.
My personal tip is Gangplank top. You can play him mid and top (you should still pick 1 of them as your main lane, but it solves the issue of choosing your 2nd position) and he is complex, mechanically demanding (tripple barrel and comboing in general, input buffer etc.) and has a certain flair. He allows you to stick to a very disciplined playstyle (don't go for kills, learn lasthitting and stay back) while also giving you the power to solocarry games (you can waveclear when behind, oneshot teams when ahead).
Focus on not dying first and farming second, the rest should come on it's own.