Is it really possible to always be able to improve?

I feel like my game knowledge is as high as it can go (before you start raping the keyboard understand that I'm talking about the basics and the advanced, not the super-advanced knowledge like "You should do an auto in X-case before you do the Q ability, so that you can maximize the W damage" or "Taking the tiny krug will give X champion an advantage as soon as they hit 7, in which case it's better than Y champion's level 7, so you should go ham but not too ham because ajslfja;fd.), and my mechanics are now as high as they can go.

Like I don't know if it's even possible to get better with mouse accuracy. I always take the little tests/training-games for it online, and they are always around the same, sometimes spiking and beating my record. I can't pin-point click a minion by dragging the mouse across the screen and stopping right at the minion. I always go too far or too short of it and have to adjust to click it. I'm not amazing. But I know how the game should be played at least in the Plat level, and my mechanics would be Gold-level I'd say.

But now.... I don't feel like I can improve anymore. I feel like the improvement curve for the game has stopped and I always play the same game every game. I play the same I have played 300 games ago and nothing changed. The point of this is, I don't even know IF it's even POSSIBLE to get better anymore for me. It seems like no matter what different mindset I have, no matter what I focus on improving, I see no improvement. In fact, instead of climbing the ladder to reach Plat, I have plummeted down into silver and got demoted from Gold 3 to Silver 2 (almost 3 now) within a month or so. I know I'm not diamond material. But is it even possible for EVERYONE to be able to learn this game to that level? I have played this game since end of season 3 and am a gold player. But someone else is Master. I just... I don't know. I'm lost.