Don't put all your eggs in one basket- Don't let winning be the all consuming reason why you play. This is a sure fire way to tilt yourself. Instead, focus on little things you can do every game to improve. "This game, I will CS like crazy". "This game, I will protect my ADC like crazy". etc. These little things are sure to improve your chances of winning, and are lest tilting if something goes wrong.
Don't focus on others- You can't control your team mates. All you can do is keep focusing on yourself. Bad game, oh well, move on.
Do give the benefit of the doubt- Seriously. Sometimes your team mates are actually BETTER than you, but they just so happen to be going up against that Diamond smurf on their way up. It happens. I find that thinking my team mates drew the short straw makes it easier for me to play than thinking they're just bad. Sometimes hearing "Don't worry, fam, I got you" helps your team not tilt and focus, and then you win. Empathy, man. It helps.
Do use the /mute all- Sometimes people are just toxic. Mute them. Focus on your game. Move on.
Finally, if you find yourself starting to lose your cool, take a second to recompose. Next time you're at the fountain, take a few seconds, breath, tell yourself it's cool, find the next thing you need to do, and go do it. Giving yourself something to focus on other than negative thoughts helps you focus.
Good luck.
WALL O TEXT CRITS FOR 9999 Damage!