You have a couple options here:
Take a break. This is the easiest one. For the next 1-3 days, whatever time you'd spend on League, go hang out with friends or bury yourself in a hobby like lifting or swimming. You should have better mental once you come back. If I were you I'd do this.
Play different champs. Surely you're not a one-trick and you can play other heroes proficiently too? Play those for a litle bit - spamming one champ for too long can bring into autopilot mode (which is bad).
Smurf. I hate to say it because it's super inconsiderate to lower elo players trying to climb but smurfing is also a pretty good way to break out of tilt. Even if your mental is total dogshit, if you're high Plat chances are your mechanics are alone are good enough to stomp Silver games. Tilt is all about loss of confidence, stomping weaker players will remind you what it's like to win, giving you some of that confidence back.