Need help getting better!

Hello,

I would like to ask you good people how to actually get better at the game. Here is some bio. I 've playing the game since beta, but always with big breaks over the course of the years. I know almost everything about the game, from lore to mechanics and in the last 3 seasons i managed to get to G1/Plat5 every season, without giving it my 100%. I believe that if i did my best,i would get plat3-plat2 tops. Thing is i want to actually reach high diamond at least, but how can i achieve that? Every bit of advice is appreciated. Sorry for my bad english, hehexd.