Spend some time thinking about what specifically tilts you. Is it arguments with teammates? Poor play? Other? And then think about what it means to play in a competitive setting where you are expected to lose half of your games.
Chat: If its a chat problem to begin with, muting is your best friend. You cannot be tilted by allied chat (or enemy chat, which can be permanently disabled in your client) if you never experience it in the first place. You can mute the pings, too, for a full array of blissful silence.
Some people will argue that you shouldn't mute people because "they might give important game information. The game needs communication to succeed". But they're wrong, because being tilted is far more detrimental to the team's chance at winning than the improvement to win chance that any hypothetical information they might provide causes. I'm pretty sure that was correct syntax.
Some people will argue that muting all is worse because "you'll mute non-trolls who might have good calls". That's less wrong, but still wrong nonetheless. Is that incorrect usage? I explicitly describe something as 'less' and 'nonetheless' in the same sentence. Idle thoughts. Your own play should be the most important thing to you.
Teammate play: Think about this for a second. Let us suppose that in a given losing game one or more players are the "cause" of the loss. With 4 allies and a solitary you, your allies are somewhere around 3-4x as likely to be the "cause" of a loss as you are. That's normal and expected. Note that sometimes nobody is responsible for a loss, or sometimes all five players are responsible for a loss. And sometimes you'll win in spite of a struggling ally. Or two. Or four.
There are plenty of reasons for a teammate to do poorly, or for you to do poorly, and most of them are not their own fault. They could have been counterpicked. Or been matched up against a player just a little bit better. Even LCS players sometimes experience a Flame Horizon. Maybe they made a misplay, and couldn't recover, or got outplayed, and couldn't recover.
"Some things are in our control and others not. Things in our control are...our own actions. Things not in our control are...not our own actions.
The things in our control are by nature free, unrestrained, unhindered; but those not in our control are weak, slavish, restrained, belonging to others. Remember, then, that if you suppose that things which are slavish by nature are also free, and that what belongs to others is your own, then you will be hindered. You will lament, you will be disturbed, and you will find fault both with gods and men."
Epictetus, from the first lines of The Enchiridion.
Translation: The more you worry about what other people do and what you cannot control, the more tilted you get.