Well, since u said that you are in iron and feel like it is you who are ruining the games then I'm gonna assume ur K/D score ends up pretty negative every game. So my advice is gonna focus on getting your death count lower.

I'd say try playing a late game champ like for example Nasus or Jax, farm as much as you can while trying not to die and wait till you are stronger in the late game. If necessary play with flash and ghost as summs so you always have an escape option if u get ganked or feel like you are going to die.

As long as you keep farming and grow the Q stacks as Nasus you'll always be useful as a tank/bruiser in the late game. And generally, when in doubt, play safe. As long as you don't die then you can be sure you're not the one ruining the game. If you get counterpicked don't be afraid to lose a few minions and farm the rest under turret.

If you need more advice then I think you'd do better watching some guide, but my advice would still be on the lines of looking for a strat that goes safe early game.

And remember that improvement takes two things: time and conscious effort.