(I'm French and trying my best to answer)

I'm diamond, was almost master, and tbh I carry a lot of my games as Support.

For exemple if you play smart, you don't greed, if you have an excellent map awareness (you see the future gank in another lane, you see your jungler trying to counter jungler or getting counterjungler etc.), if you know who to protect and how to do it, then you can make a huge difference.

It happens a lot that, yes, I can't really do much because the others are feeding and even if I'm an excellent Nami, it's hard to win when the enemy Katarina already has 4 kills. And sometimes I really can't do a thing because I'm playing alone while everyone is toxic, trying to ff and whatever. But keep doing good at what you do, don't flame or waist your time with the chat (unless you try to motivate your mate, what I like to do lol), and you can still try to win the game by the end.

I won a lot of " losing " game like that, whereas a lot people wanted to ff fast. Remember that even in high diamond you can always manage to win the game because people always make a lot of mistakes. Even in my team when we're winning so hard people are less careful and greed more, which can cost the game...

It's almost never a losing game unless you want it to be.