So, I'm a little late to the party, but, oh well. Won't stop me from offering what help I can.
To preface, though, Prandine and Jo0o are pretty good people to defer to for this - while Jo0o's initial response was a tad on the sarcastic side, there is plenty of worth in what he says after that first remark. In fact, I'll add to it and say this;
If you still want to play online with other people, I'd personally suggest at the very least to look into Cooperative Multiplayer games over Competitive ones - but, those are relatively few and far between, so you may be best off looking for Single Player experiences.
So; more to the question of reforming. The reform process will virtually always start from one point; Identifying the problem. Once you identify what the problem is, it's just a matter of figuring out the steps from there.
If you have problems with anger - you can try redirecting that anger to a better goal, or practising temperance to mitigate the impact of that anger. Breathing exercises, meditation, various other outlets - y'know, punching pillows, squeezing stress balls, fucking around with a fidget spinner, etc.
If you have problems dealing with other people - again; breathing exercises and other such outlets can help. Beyond that, you can either work to adapt your behavior to be a bit more forgiving to those people (which, IMO, is too hard of a route to follow), or you could work on some preventative behaviors so that you don't wind up saying anything to them, no matter how they piss you off.
One behavior I've developed in that regard is constantly rethinking and rethinking what I'm about to say. "Isn't this an insult?" "Should I really say it?" "Is it worth it?" "Can I say it any better than this?" "Do I even have to say anything at all?" - etc. Anything that survives that rethinking process typically is pretty tactful and safe in general.
Although, all this is said with one stipulation; the reform process may start with identifying the problem, but it only truly begins if/when the person in question has the will to start working on it.
So, all said, the reform process starts with you; it's not easy getting on your feet with it, but it's definitely rewarding in the long run once you do. My advice is pretty generalized, I'd say, so I would also again advise to think about Prandine and Jo0o's advice in particular, whether playing Vs. AI matches and a little temperance might help, or if you may just need to call it quits in League and find greener pastures.
Nobody'll fault you for whatever you choose. Best of luck in your reform process, and I hope what I said helps at least in the slightest.
