First and foremost; breathe. Four seconds in, seven seconds out. Breathing exercises help you focus.
Second; embrace the possibility of losing (even if it is ranked), but challenge yourself to win. Yes, intentional feeders/trolls will probably fuck you over and make you lose, but you're good enough (by your own respective merit) to win the next match if not that one - and your skill will even it out.
Third; find an external method of venting; punch a pillow, yell and swear out loud, squeeze a stress ball, fuck around with a fidget spinner, etc. - short term vents of your anger to help mitigate the harm that anger can do.
Those are pretty much my three best bits of advice, aside from the obvious of "mute and report" - muting may not stop them from intentionally feeding or trolling, but it will at least prevent them from goading you into flaming them, and reporting will make sure that their behavior gets reviewed and possibly punished.