How to avoid throwing games

I recently noticed that I do fairly well in early game throughout mid, usually dont lose lane and manage to pull of some successful roams to top or bot and this goes on through mid game and it really seems like we re going to win but then somewhere something goes wrong.... I looked at my most recent ranked match with kassadin against a viktor which went quite well for everyone on my team but then somewhere along the way we lost. Is it because their champions scale better than ours? We managed to secure all the drakes and also two barons but just cant do anything with it. Admittedly I also felt quite useless because in teamfights i could always only clean up, which i guess is kassadins job but... I tried splitting top and also got a turret and almost the inhib turret as well but then has to back before the enemy wrecks my team. How can i manage to not throw late game? What are things I should pay attention to to avoid these things? If you should have a look at my history, its mainly my ranked games with kass that I struggle late with, since jungle i play tanks. Here the match history
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