I understand your problem and I have suffered from the same.
It's not "maturity", we are all different people.
Anyone who thinks EVERYONE should respond the same and be able to control their emotions is the same kind of person who would say someone chooses to be gay.
If that was the case everyone would act the same and nobody would have emotional diversity.
I feel it's kind of sad Riot doesn't recognize this and give us the option to disable our chat interface OUTSIDE of the game and not be able to enable it until we are back out.
This would help allot of people and be the easiest way to give players a simple effective tool to reduce toxicity.
I have anger issues, and many more but never have I said to myself "i want to go in this game and be an asshole", I've never thought that ever.
I don't say hateful, homophobic or racist things but I get upset with players who refuse to listen when I try to help.
It's just an emotional response I have, I have to type something, I can't control it.
Going to any real life sporting event I'm so used to trash talk I find it strange that this eSport is expected to behave like we are not on the internet.
Also the target age group of players are kids who are emotionally not always in control.
So what to do? Play every game like it's your last chance, because it might be.
Don't enable "ALL CHAT" ever, its just another chance you might get pissed.
Accept that you are going to lose, you will lose, you have to lose. I'm a sore loser and accepting defeat doing the best I can and feel my team wasn't trying is so hard to deal with.
Just focus on improving yourself and keep notes on where you feel the game went south and what you could of done to change the outcome.
Most of the time you can't do anything and it is what it is.
Keep your game solid, looking at your winrates with champs and playing more champs that can do more then just win 1 v 1 if you have a team to carry.
There is so much I could keep going on about but this is getting long.
Until riot will just let us disable chat we just HAVE to keep cool, play champs that are fun and hold back outbursts the best we can.