Closing out a game with Lee Sin

Lee is one of my favorite champions. I'm currently Gold 1 and I'm just struggling with winning games with him. Please don't anyone come in here and tell me he's garbage and I can't win just deal with it because I know that's not the case. I win pretty much every early game either getting fed or getting assists but in teamfights I'm having problems winning no matter how far my team is ahead. I know theirs something critically wrong but I just can't pinpoint it. Any suggestions?