I think most people do this often, playing on auto-pilot. I mean it may sound a bit cheesy, but make sure you're in the right state of mind when playing the game. you don't wanna be playing seriously while tired, hungry, or have something on your mind, e.g. work you need to do. Make sure your hydrated, go fetch glass of water as game loads . Don't be alt tabbing to browse while waiting to load in etc. make sure you are 100% focused on the game.

For all I know you're doing this already, but you want to be keeping up with the pace of the game. as soon as the champs are locked in you want to be thinking about their abilities, comps, globals to watch out for, cc chains, specific abilities, how you will play the lane, everything. If you're laning think about where the enemy jungler is starting, what kind of jungler are they, e.g. eve/lee is probably going to gank early, yi/nocturn will probably farm up. You need to be conscious of what you're objective is in lane before you get there. Essentially it needs to be less binary of what the current situation is, but you need to get into the habit of predicting the future.

I think a lot of deaths will come down to playing too aggressively without knowing the outcome of said aggressive play. When it comes to things such as knowing whether you can all-in someone/ should just poke/farm, it comes with experience. But if you want to actually find the limits of your champion, you have to jump in normals and e.g. dive in on any opportunity as Riven going 0/20. Watch VODs of how the champ is played. If you're playing a game and you want to win however, you have to play ONLY within your limits. If you don't know the outcome of an aggressive play then just don't take it, focus on other advantages such as CS-ing, making sure your lane is warded and safe if you're pushing.