I've got an irl friend who actually developed anger issues from League, often after playing customs. He resolved his issue by no longer playing customs (with other friends that is) as well as ignoring toxic players, or disregarding small mistakes he'd used to get worked up about.
If you like League of Legends, but your anger issues are getting in the way of your game-player, resolving it has to come from you. Idk what it is you get frustrated about, but if it has something to do with unskilled team-mates, or trolls, just know it's something everybody has to deal with.
I can't help you too much since you weren't specific to why you get mad, but I hope you get it resolved soon and realize that you often play a lot better when you're in a calm state of mind. Anger often leads to bigger throws.
Good luck!