Losing Games but still carrying, what do I do?

before I start this, I wanted to disclaim that im not trying to blame my team for losing. or imply I should be ranked higher. I just want advice to improve :)

So I was in a S5 game, I was diana against a tf. I'm a very experienced diana (I have over 300k on all of my accounts on her). Its not rare for me to carry games with her; In fact I usually do. In this specific game, I got really fed, probably more fed than anyone else on their team. However, Top renekton lost really hard to Trundle, and my MF/Lulu got destroyed by ashe/sona. My kayn jungle then started feeding all of a sudden. Although they were all really fed, I was still ahead of most of them.

My problem lies here. What should I have done in this situation to carry even harder? Note that this was my promos. If i went to splitpush, my team would have engaged in 4v5 and died (it got to the point where they couldnt even hold a turret without me) - so I couldnt split. Instead, I decided to participate in team fights. the problem is, my team was never grouped, and when they did group, i would get focused down quickly and we'd lose the teamfight.

Can someone give me advice on how I can do better in games like these? really disappointing to lose promos because people couldnt play safer.