I really need help understanding something please

So lately I been trying to play different champions, like Riven for example. But I can't seem to be able to play her well, I know how her kit works but I just can't make use of her effectiveley because I can't get to enemies due to being kited to well.

I'm not doing so bad to the point where i'm feeding but I would like to improve if possible.

I know she isn't an easy champion to learn but is it possible for me to get better at her? Or is it a case where X champion is not for everyone?

I am really good at playing champions like Kai'Sa, Soraka (pls don't laugh at me lol), Anivia, Cassiopeia, Jhin, Yorick.

There's one other matter. Lately I noticed that I can't play league normally, i'm always feeling anxious and really un-relaxed. At some points I start to shiver/shake (not in terms of cold) for no reason. That is also one of the reasons why I stopped playing ranked.