you should try to focus on a few champions only. I play adc right now I play either Ashe or Jinx only, altho I did pick sivir a few times but I avoid it because I'm not enough experience with her. if I don't get ADC, I try to take Maokai or Wukong top because I have a lot of experience with Maokai and both of those champions are meaningful if I'm bullied out of CS in lane. Their ults really help whoever on my team is strongest to have more impact. I can jungle with wukong too. I hope I don't get sup or mid because I don't have any champions I'm experienced with in those roles, but usually I get adc or jg or top.
I've played Sivir maybe six times in the past three weeks at most, and it's very clear I'm not used to CSing with her or managing her mana, and I miss her boomerang a few times a game, additionally I'm not familiar enough with her damage spikes. it's a very different story with Ashe and Jinx, and I know that I'm more likely to win with the two champions I've played many many times this season. even in bad matchups like against thresh, I'd rather play those champions on the blind. if I know I'm against a thresh I do take Sivir, but on the blind it's Jinx or Ashe every time. usually the one I just played last game. but if I know the jungler or mid laner, I might pick like Ashe against Yi or Jinx against Yasuo, for example.