how to get an s without playing support???

I can not get an S to save my life. I've been 18/4/17 and got an A. I farm all game, push objectives, focus the adc, ward like a support, micromanage my positioning.... i can't get an S. I see supports (aka my gf) go like 1/4/17 and get S+ and i sit there and scratch my head. At this point i'm about to take rumble support just so i can cheese some S ranks. The only way i ever get S nowadays is in aram and 80% of the time it's a champion i have like level 2 mastery on so it means nothing.

TFW you finally play a game flawlessly and see that sexy curvy S in the future the other team forfeits and you get a B.... or die ONE time back-dooring an inhibitor and get an S- (feelsbadman)

Someone tell me exactly what the system is looking for to give me that luscious S. Its sad when you have 150k mastery on someone and they are still level 5.. Is it required to die less than 1 time in the hour long game? do i need an average farm of 12 minions a minute?... i don't understand where I'm going wrong, just me?