win lane every game, go positive and still lose to inting teams

So this is my first season of league and i started off in bronze ( as expected) and then eventually got my way out of bronze to mid silver. I'm currently s3. I play mid lane and i started by maining Anivia which i was pretty good at and had an 80% win rate at one point which was fine. Then i got to silver and i could no longer carry my team. I would just lose games and i didn't know why. So i started to pick up another champ that i could carry on and i decided on cassiopeia. This was extremely successful as i hardly ever lost, only when i got auto-filled another lane. This got me up 2 ranks and into s3. This is where im at now. I got into promos the first time for s2, one the first and then lost the other, got back into promos and lost both games with the free wins so i got pretty tilted as you coudl imagine. Now i essentially win every single lane on cass and go positive and i just somehow lose but i look at my teams score and they are always doing extremely bad and i always have the bets cs int the game.

I don't know what to do, or what i'm doing wrong but someone please help bcoz i'm losing my mind and i'm soon to become a toxic player if i keep getting "trash" teams. On that point, i have never done anything extremely toxic and i have never purposely thrown a competitive game such as LoL, i straight up don't have the audacity to do it and i tilt the most when other people do it.

P.S I've linked op.gg for reference make sure u use ranked solo bcoz i do play norms with friends. Ty if anyone can help.