Tldr; switch to norms or ARAM when you're tilted. Play a different champ/build path. Don't expect to win or have competent teammates.
Hey Primordial,
It's really great to see a player come here and ask for help before it escalates to a permaban. The best advice I can give you is something that has been working for me lately. I've been having some real world issues that have put a lot of stress on me so lately I find myself getting frustrated SUPER easy.
I find that most of my frustration comes when my expectations aren't met. IE; we have an OP team comp so we should totally win, then we don't. We broke an inhibitor and took down Baron and have all 5 members and a 10k gold lead, then lose. My jungler is competent, but then does a drive by without ganking enemy mid-laner at 15% hp who is overextended. I'm good at X champ, then proceed to go 0/4/0 in laning phase.
These things tilt me so bad, so when that happens, my next few matches I do something that I have no expectations on at all. I will play a new champ/role/build in norms. That way I have no expectations on dominating lane or anything like that. I will do something that makes the game how it should be played: fun.
Also, try out ARAM matches when you're feeling frustrated. Just don't expect to be on even footing with the enemy team in case they get those OP ARAM champs. When I see our team of Udyr, Rek'Sai, Kalista, Garen, and Poppy against an enemy team of Ziggs, Caitlyn, Varus, Nautilus, and Lux I know this shit's over before I even load in. But hey, you just play to have fun and winning is just a byproduct of that fun. At least that's what helps me.