I cant win games no matter what i do,plz help me

I just cant win ranked games,every game is a coinflip,after my placements i got bronze 3 and climbed to silver 4 and now i completely stuck,sometimes no matter how hard i try i cant win games,especially i dont know how to close and end games that my team is winning,i know the golden rule objectives over kills,but somehow even i get feed and win lane i just cant carry my team and end the game and win,look at my last game with Jhin,me and Pyke destroyed them botlane i was 9 0 and when it came to teamfights we just couldnt win their renekton was so tanky and strong and i didnt build mortal reminder i wanted to buy that next we just couldnt win 5v5 i focus their carries and kill them but that didnt help i just got focused so hard rekenton and jax just flashed on me and pyke couldnt do anything to protect me,i think my team could have pealed better for me but i am not blaming my team bcs i should have carried,i coulnt i belive we didnt win this game,i know my farm is abysmal in some games but i can improve that ,the deal is i dont know how to end games i could really use some tatics how to teamfight how to splitpush bcs i play a lot of nasus also,when to group when to rotate to my team,how to ward better and when to take objectives. I know that still have a lot to improve on, i started playing in december and i am suprised i am silver considering i am a new player,Sry for the long text any help and advice is aprreciated.
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