You need to relax and realize that things like games aren't worth the least bit of stress, it's seriously just a video game. You lose a game and what happens, you lose some LP, get less IP, and gain a loss? Some stupid stats in a video game are meaningless. Yeah it's nice to win and gain LP and rise through tiers, and that's the goal of the game, but the purpose of the game is for fun and enjoyment.
The best way to rage less in game is to relax yourself before the game and think about how you want to act. Think "I'm going to focus on my own mistakes, ignore negative comments, and try to have a supportive attitude", then follow through with that. If you're not reminding yourself this before every game then you're going to act how you usually do without thinking. After so many games your behavior will start to improve, and your gameplay. Say "GJ" when your team makes good plays, and don't say anything negative on bad plays, a player knows if he messes up so there's no need to say it in chat. Stay focused on the game, not the chat, ongoing conversations and arguing is worse than an AFK/feeder, it seriously is.
If you want to get better, the first step is to relax and follow what I said above. Once relaxed, focus 100% on every little thing that you're doing wrong and what you could be doing better. Maybe you're not last hitting good enough, maybe you didn't use your trinket enough while off cooldown, maybe you didn't pay attention to the minimap while you got ganked or your jungler got invaded, maybe you could watch the minimap and use pings to help your unaware teammates see the incoming gank, etc. It doesn't matter if your 4 teammates fed all game, you probably couldn't have changed the outcome, but you surely could have done something better, and when you realize what you can do better you can apply it to the next game. You're playing to make yourself a better player, not your teammates, so don't focus on what they're doing wrong.
As for your general attitude outside of League, I can understand you. The first thing you need to is realize what's important in life and what's not. Are games worth any stress? No. Is what immature people have to say worth any stress? No. Important things are your health, family, and friends. Nothing will change in your life or get better if you don't make it happen, don't sit around expecting your life to just change automatically.
This is something that helped me out, and I repeat it daily, regardless if you're religious or not, the main point is to understand and change your attitude and perspective of life. "May God grant me the serenity to accept the things I can't change, the courage to change the things I can, and the wisdom to know the difference." Basically you have to understand what is out of your control and have peace of mind that it is what it is, and you have to understand that nothing in your life will change unless you take action and do what you have to in order to get where you want to be.
Relax, enjoy League, enjoy life.
