Its tough. Ive always had a temper, and this game exacerbates it like none other. I can go play CS:Go, TF2, basically any other game but this game just pushes my buttons.
For the most part I dont get mad at my teammates poor plays. I get mad when I get baited by them. Like, when you know its a bad idea, or you want to back, or you want to do anything else because you know its a bad idea, but you follow through anyways like a good teammate and get baited. That just sets me on tilt and rage mode fast. Also, playing team-oriented builds and roles and getting screwed over by people who dont care and do whatever they want.
I get out of this occasionally by just doing my own thing. Stop playing full tanks, and pure supports, stop getting baited by my supports and junglers, and generally focus on having a good game or wrecking some people to the beat of my own drum. As bad as it sounds, I am worse when I try and be a team player. Its no surprise that I do the best when I first come back to league and I dont rage, and I dont care what happens, I just do my own thing and work on getting my abilities back, and I magically play 100x better in that first month or so than I will the next 3-4 I tryhard in. I really noticed this when I made a smurf to play another role, and I cared so little about winning or what my team was doing, and just did my own thing top lane with cheap champs trying to learn the lane and got an entire division/tier higher than on this account. Kind of opened my eyes. But I still rage from time to time, and I am trying to not rage.