How can I spot my own mistakes and improve as a beginner? I felt like I'm progressing but now I'm not sure.

Played as Sivir ADC last night and got abused by the whole team because my Thresh sup kept dying. I even saved him with my ult by giving him the speed to escape an encounter with a slither of HP but he ran straight back in and died.

Thresh was around 0/11/17 and I was something like 7/3/12 so I feel like I wasnt that bad. We ended up winning but the whole game everytime someone died it was my fault according to the chat, even the top lane. Made me wonder if there was an aspect of ADC I was missing or just straight up getting wrong.

The main rage comment I got was that I was just letting Thresh die and I wasn't helping. I don't see how I could've helped when I was making my way to tower after recall and he decides to basically suicide 1v2 in bot multiple times. Can Sivir help apart from pushing and acting as a threat?

Was I not pushing and being aggressive enough? As far as I know Sivir is generally pretty weak at the start but carries end game which is what happened in my game. I tend to poke even when I play more aggressive champs, was this also wrong when playing Sivir?