Odds are you made a few mistakes yourself. Typically when I lose a game where I am dominating like this it is because I make mistakes that cost me the game. Your ahead and you need to use this to help your team. Yeah not every loss is your fault when your ahead. Sometimes there are teams you just can't do anything about.
I played a game a while ago where my top, bottom, and jungle were not doing well at all. I was super far ahead on Akali and dominating my lane. The only reason we didn't surrender at 20 minutes is because I was able to win us teamfights, but when most of us were dead or on super low health after we weren't really able to siege anything. At one point in the game a teamfight broke out in which I would have been able to get a penta kill (I still haven't gotten one), but instead of getting the penta kill I allowed the Vayne on my team to get most of the kills. This got the Vayne a lot of gold and xp, and because she got so much from this she was able to help me help us win teamfights and very soon after the game.
You don't need or want a ton of kills if the rest of your team is behind. Just because your 18/5 doesn't mean you can 1v5. Give some of those kills to the other carries on your team. Get them gold, so they can start doing things in teamfights other than dying and running away, and you will find yourself winning more games even though you have a lower kda.