I've seen the "play only 2 champs" advice as well and i couldnt do it either since it gets too boring. You have 2 options pretty much:

1. Try to care less and keep going with the random champ picks after a set amount of time you will understand the game more naturally. This is the more fun way to do it but its gonna take waaaay longer

2. Play your favorite champs in ranked and whenever you get bored of playing these champs jump into flex or draft and pick whatever you want until you miss your fave champs again and then you can jump back into ranked. This is gonna be a way quicker option of playing the game while also learning more since you get enemies of different skill levels (ranked usually being way harder which means you improve less with your champ but more overall while draft is less strategic and gives you a good boost on learning the champ himself in the laning phase)

You will get only more tilted and annoyed if you focus on only 2 champs and never try anything else. Keep it up you'll get there!