How do I become nontoxic after being toxic for so long?

Stop caring about how your teammates do. Yeah they're gonna do a lot of stupid shit, learn to sit back and relax and enjoy the show. You probably do stupid shit too. I know I sure do.

Just follow the old good rule: "if you got nothing nice to say don't say nothing at all".

That's it - we can't really change your feelings about the game - but this should be enough for you to not be banned, chat restricted or miss out on rewards.

Leave the game for no less than two months. Don't think of it as a ban or suspension. Just use it as a mental vacation.

Saving this page, helpful advice overall. I'm starting become more toxic, and I hate what its doing to me and my reactions towards others.

Turning off all chat is a good place to start.

just learn to take what comes to you in game instead of getting mad at people for what they did wrong or what they didnt do. if its them being toxic, just ignore them or try to reason with them(probably wont work), try to work with your team to the best of your ability, good communication is helpful too. and one of the biggest parts, be friendly. just try to be nice and you'll be less toxic

The first thing to think about is what exactly it is that makes you react that way. Every time it happens, stop for a sec and try to figure out exactly what it was. Was it an ally's poor play? Was it what they said? Is it certain champions? Is it dying in certain ways? Once you figure out WHAT causes you to become angry, it can be easier to try to avoid it.

But... the biggest thing is a shift in how you approach the game... I've noticed most people who get toxic unintentionally do it because they're focusing so strongly on winning that they just can't keep themselves calm. A while back I posted a thread with my advice about how I approached ranked play, the 'rules' I followed to help keep myself calm and having fun while ensuring I kept getting better.

I'll share it for you since it might help: http://boards.na.leagueoflegends.com/en/c/miscellaneous/Xp3EhQMY-advice-for-enjoying-and-succeeding-at-ranked-play

But all of the advice here is sound... even if most toxicity starts within your team, not the enemy team, so I don't know if turning of all chat is going to have a huge impact. Aside from that though, everyone's advice here is pretty sound.

Also remember that this IS a game. League of Legends is, until you're making money, just a game... and games are supposed to be fun for everyone... so have fun with it.

Either learn to deal with it, just not caring, or play with the others, no matter how much you hate it, if you see a defeat, all other 4 will see the defeat too, also DO NOT RAGE QUEUE AFTER DEFEATS, playing another match after raging, will NOT help you

Things that have helped me:

1. Listen to music. Find something that you like that you think of as soothing, neutral, or happy/peppy; let it run in the background while you play. Music does a lot to shift emotions, so it can help keep a frustrating moment from turning into a rage-face moment.
2. Play less. I don't mean take an extended vacation from the game - though that may certainly help. I mean don't binge play. Take a break between games, even if it's just 5-10 minutes to catch a breather, grab a drink/snack, dance around, or whatever. This will help you relax again between games. Even if it was a good, fun game, take a break. The most enjoyable games are full of tense moments, and releasing that tension between games is a huge part of keeping your cool. An extra side-effect of taking short breaks is that you stand a better chance of avoiding getting stuck with the same people that may have irritated you in the previous game.
3. Check your baggage. This ties in with #2, but I think of it separately. Unless you're at a very high tier, you're unlikely to play with the same people more than once or twice. So it's important to remember that what happened last game has nothing to do with this game. And yeah, it may be another last-pick auto-lock Teemo jungle, but don't blame this guy for what that other guy did.
4. Remember why you're playing. There's a million reasons people play League, but all of them that I've seen can be boiled down to two primary goals: having fun or getting better. I'm going to assume that you're playing to get better, because the ones who just want to have fun don't tend to be toxic in my experience. So remember that. If a game's going bad, you can still learn from it. I'd wager, in fact, that you may be able to learn more from losing than from winning. And if all else fails, try risky stupid plays... you might pull off something amazing and turn stuff around, or you might learn how not to do that kind of play.
5. Play with friends - and use voice chat. If you're playing Ranked, find a duo-queue partner. If it's other game-modes, bring your friends along. Work out a voice chat system. You're less likely (in general) to want to bitch out your friends for their play, and less likely to say things to any randos when you can just yell it into your mic. And it's always calming and gratifying when you bitch about something - "What the fuck, Kat, why were you even fucking there?" - and can hear a friend agree - "Right? Dumbass." Using voice chat allows you to keep your chat - and account - clean, while still venting to people who, ostensibly, won't judge you for it.
6. Use your mute button. The exact second that someone says something that's past your threshold of acceptability, mute them. Seriously. The goal isn't to keep yourself from getting offended, but to keep you from getting angry and retaliating.

It's not a comprehensive list, and you may not be able to use all of them, but hopefully it'll start you thinking down the right path.

Additionally, Eleshakai's post has some fantastic advice, and AFK Farmer is very right about turning off /all chat.

Remember, it's not about pretending everything is unicorns and rainbows. It's just about avoiding acting like a jerk, and while that can be hard, it's very doable.

You could stop being an asshole

In place of any toxic chatting, replace those words with Radda, and nothing but Radda. You can go all caps radda if you need to "RADDA RADDA RADDA RAAAADDDDAAA!!!" If you need to, for those who know Schnitzel it'll be a good laugh but the caps and exclamation points still get something across without being outright toxic.

It's really difficult. It took me being banned to realize that I need to stop being a douche, and I have still been having troubles lately.

For instance, my first game today was vs a Brand as Heimerdinger. Not only is Brand a horrible champ to fight, I also got counterpicked. I got destroyed in lane mostly because of Sion's amazing ganks and Brand's 100-0 ability with my turrets, and I was pissed. The reason I was so pissed was not because I lost lane, but because our Diana outright REFUSED to gank. I asked her when I was at 0/0/0, 1 death, 2 deaths, etc. She didn't even acknowledge me. All she did was camp top and fail gank after gank while our Renekton fed his ass off. The whole time, I begged for her help and got none.

My second game was even worse. I made the mistake of playing Janna and not being able to predict the stability of Riot's servers/my connection. The whole game, I was plagued by lagspikes that hit REALLY hard and then vanished. Worse yet, our Vayne gave first blood (>Vayne >SoloQ >VAYNE >FREAKING VAYNE) and then said "bot's over". I got a kill lined up for her and she didn't even try to get it, so I became 1/0/0. Because laning phase ended so early, I wasn't able to get any assist from bot and was gold-starved as fuck. For the rest of the game, she farmed Skarner's jungle and tried as hard as she could to avoid teamfights. I'm not exaggerating, she sat in base/Skarner's camps for the ENTIRE GAME. It was essentially a 4v6, with her Eing Trist to safety and giving her gold. If I recall correctly, she only made a teamfight appearance one time, only to Condemn Tristana to safety. I kept dying because of their CC and mobility heavy team comp and poor positioning. The constant lagspikes every time I would try to get near anyone didn't help. She ended the game 2/1/4 and did a whopping 7.5k damage as an ADC. I lost my cool at her (Not swearing, just saying things like "You're not doing anything, please do something, we NEED you, etc) and so did the rest of my team. For bonus points, toplane insta-locked Teemo and said "Don't worry, it'll be fine."

No. Trading a kill for a death the entire game is not fine. Playing Teemo is not fine. It's always the Teemo and Vayne players, man.

TL;DR- If you see a Vayne or Teemo on your team, DODGE ASAP. The only reason I didn't was because I had already dodged twice and didn't want Lyte's statistics to ban me. Never play a PvP match in the early season, you will want to kill yourself.

Jubbinaut said most of what I would have, but I'd like to add a couple things.

1. Waiting between games helps both with resetting mentally and with letting the queue move on so you don't get paired with the same people again.

2. Watch for reasons to say thank you or good job to your teammates. If you're used to watching for mistakes, it'll be easier to replace what you're watching for than to completely stop i.e. instead of complaining about a gank that left the enemy escaping with a sliver of help, say thank you for helping to take some pressure off your lane.

for me its the other way around. this seasons reset is turning me into a rager!