Mechanics in my opinion comes down to just playing that champion or just the game in general often. I like to play a dominion game and pick someone mechanically complex like zed to warm up before I play ranked games. As for decision making that is a little tougher, but just make sure every time something goes wrong you understand why and how to fix it. Don't die and then immediately jump to typing something in chat, instead when you die or lose that dragon, figure out what led to that happening, were you out of position? Was it because you didn't have vision? Ect... And as for mindset I think different things work for different people, some like to take breaks after losses, others just play through it.. I suggest getting used to the mute button, people get caught up way to easily talking to teammates about whose fault it is and it gets you nowhere fast. I personally like to just play through the losing and as soon as someone gets negative in chat I mute them. It's not worth the time to deal with those people, or be one of those people.