This is part rant, part observation, and the contents of this blog are pretty obvious to some, but I think something like this is good to inform those who get really heated when playing. It will have a bit of a story from a recent experience, so I'll include a TL;DR part as well.
I'm like you. Or, maybe I'm not? I'm generally pretty competitive. I've had my fair share of playing games competitively, from Counter-Strike, to top PvE WoW (top10 world) to Team Fortress 2 (attended multiple lans hehe hat simulator). One of the things that you do when you're competitive is that you often get caught up in the heat of the moment and lose control of your emotions and it can definitely impact not only everyone's (yourself, teammates, opponents) play, but it can leave a bit of a lasting effect on people.
A simple flame here could spread to your whole team, leading everyone to potentially start flaming the person in question. This obviously creates a shitty environment for you to play in: you forget cooldowns, you forget rosh timers, you start making really dumb decisions in the game and it becomes less fun for you and your whole team.
"Toxicity". This term gets thrown around a lot; it's practically synonymous with "being an asshole". While I'm not particularly fond of the term, it is pretty much a mainstay when referring to someone with poor behaviour, in and/or out of game. For the sake of my blog and for a bit of fun, I'm just going to refer to it as "being an asshole".
Assholes are the kind of people that get unnecessarily mad on the road when driving - honking horns or anything like that. They're the kid that pushes in-front of multiple people at the canteen line and they don't give a damn. They're also the kind of people, in videogames, to not only flame you for mistakes you make constantly and to remind you that you're "shit", they complain when nothing goes their way in game, and are generally extremely aggressive people - making the game less enjoyable for everyone else.
For a bit of anecdote - I played a game yesterday (one of my first in about 6 months). I was pretty damn rusty, been playing mostly FPS for the past few months. I'm only at 4k MMR, so it's pretty close to the 'average' game for most people. I was asked to pick a solo safe lane that was able to fight by themselves, so I chose Lone Druid. I know he got updated recently, and I've been watching games here and there - I've been out of the game for 6 months and, chances are, I'm going to be even worse than I was.
Fast forward to the game: about 15 minutes in one of our team's assholes (invoker mid) gets into a weird engagement, with me somewhat nearby. Suddenly, 4 out of 5 enemy heroes are there and he overcommits. I'm on full health, and tp out. He starts calling me "shit idiot you're on full health why?? not even controlling your hero properly". Instead of getting upset or arguing with the guy, I just typed in gamechat (paraphrasing): "Sorry team. I'm real rusty I haven't played in months, so bear with me while I get into the groove of the game". A couple of team members were like "it's alright man no sweat, you did alright in lane just remember LD is a bit stronger now so you can be a little bit more aggressive" etc etc. Asshole1 would flame me here and there for my decisions/execution, and all I'd do was shrug and apologise for the mistakes.
Further on in the game, asshole1 (invoker) gets into an argument with asshole2 (LC), and they argue the semantics of "It doesn't matter if you're good - it matters if you're less bad than the other team". A rant ensues for about 25-30 minutes (note: it turns out to be a 50 min game), and we'd already taken two rax at this point. The two assholes decide to continue arguing (asshole 2 called it 'lecturing'), meanwhile the rest of our team is just saying stuff like, "hey guys let's push and end the game", getting completely ignored by these assholes.
Long story short - two assholes extended a game unnecessarily because they let a stupid argument get between the game and the next game, and assholes just give you shit constantly. As a result, it ruined everyone's fun.
The important thing to note is, after about 25 minutes into the game, asshole1 had completely forgotten about me because I more or less didn't react the way he wanted to his flames. I took it on the chin, and just said sorry.
You have to understand that, no matter what, there are always going to be assholes. The best advice I can give to you to make sure you're keeping your head in the game is, if you make a mistake and someone gets mad at you: just apologise. Following this, ask your team for suggestions. It is probably a bad idea to directly ask the person who is giving you shit, because chances are they are just going to be unreasonable and say "uninstall dota" (or along those lines), but you can see in my above story that one of our team mates gave me a small bump in the right direction for what I was doing wrong. If the asshole(s) persist, report them. Chances are, they've been assholes to other players, so your report may be the bump they need to low priority.
I wouldn't necessarily mute someone if they're being an asshole - unless it escalates to the point of it becoming uncomfortable to you. Chances are, despite them being assholes, THEY MIGHT say something useful about the game in chat.
The final stage is you becoming an asshole. Maybe you were 2k MMR once and there were a bunch of assholes in your games. You made mistakes, assholes called you things. Suddenly, you're in the average/above average DOTA - you think you're top shit, and you become an asshole yourself. This isn't too hard to avoid as long as you attempt to look at what happened in your game positively, "I didn't engage in this fight because I thought I saw extra heroes there. Looking at the replay, I was wrong, and, I suppose I can understand this assholes' frustration".
Anyway, this is all over the place. There will be a lot of grammatical mistakes, but that's what you get with something similar to a stream of consciousness.
The TL;DR is; people get worked up in videogames. You have to try not let it affect you. Not only that - DON'T BE AN ASSHOLE! Being an asshole ruins fun for everyone!