If you're a repentant rager who is trying to quit the habit, then remember this: Losing is okay. Sure, it's not great. Nor, however, is it unbearably awful.
For the most part, nobody's trying to lose nor deliberately not trying to win. If your team isn't playing to their utmost, it is frustrating but you don't have to let it be enraging.
In fact, even if there are trolls on your team, consider the matter in perspective. Sure, they make you cross but why let them make you furious?
Moderate your response; avoid extremes. You have the final say in how you respond to a situation. Nobody can actually "make you angry." They can make mistakes or even deliberately goad you. It may be comforting to lay responsibility at somebody else's feet but in doing so, you also relinquish your own agency. You are granting other people power over your emotions. Assert control over yourself. Ultimately, you cannot assert control over anything or anybody else. You will improve your attitude, your enjoyment of the game and, I contend, your ability to affect the outcomes of your games as well.
If you want to cultivate a state of mind that makes it easier to put this advice into practice, develop some perspective.
One day this week, serve soup at a homeless shelter, pack groceries at a food bank, read to some children at the library. If you're an adult with a job, start a correspondence class in that thing you always wanted to learn but which never seemed practical. Learn to crochet, learn to play a musical instrument, make supper for your parents.
Empathize with some real people. Do some things that are really enlivening. Serve some purpose that's really important. Then, some stranger performing badly at a video game won't seem worth you popping an artery over.