Why do I suck at Kai Sa so bad?

I've played MOBAs for a while now and have my own mains and everything since moving over to league. I'm at least decent at Jihn and Twitch, but ever since unlocking Kai Sa I have not won a single game where I contributed anything of significance to the team. My best rating was a B- where my team steamrolled and I rode the wave. My average rating is a C-.

But every time I play against a Kai Sa, I get steamrolled and lose. She can't be a terrible champion and everyone I see says she is one of the best. I'm extremely frustrated by not being able to play this champ. What am I doing wrong? Why can't I at least play decently in the occasionally?