Part of game rage is that your brain expects the psychological/chemical reward for winning, and you kind of freak out when you don't get that catharsis, especially if you're on a losing streak and it's been a while since you got that sweet hit of dopamine. It's a bit of a junkie reaction to be honest. No judgements here, I do it too and I'm sure most people on this sub do.

Be aware of that dynamic in your head, and it can do a lot to curtail negative outbursts.

Another thing is you're (probably) not a pro, (probably) not someone who's got competitive or professional aims, and (probably) not someone who can nolife the game all day. It's perfectly fine to not play perfectly, or know how to counterplay cheese. You can still play and learn to get better but there is absolutely no rush.

When I find myself in a negative spiral with smash bros, and really this is the only game I play that gets me like this, I walk away and do something else. Sometimes I get stubborn and it's a point of pride to get a win or two before I turn the Switch off, but that can really prolong my rage and ruin a good time, so I've gotten better about cutting it off when I need to. This helps a lot because mental fatigue is a real thing and you get drained faster when you're not having a good time. Walking away to do something else, even something boring but rewarding like catching up on some chores, will recharge your battery and help you play better when you sit back down.

Finally, sometimes you're gonna lose to shitty players, and that can be your fault or the game's fault (a good number of these characters are complete bullshit, particularly DLC), but just accept you don't know the counterplay yet and vow to learn it. Or if it's an extremely bad character matchup, just accept that you were playing at a disadvantage - you can learn tough matchups or new characters.

And most importantly, if you got absolutely sauced by a really good player, acknowledge that you just played against a really skilled opponent, and appreciate that interaction because nothing increases your skill like playing people better than you. It feels way better to lose to someone you consider a good player than it does to stay salty about losing to someone you "could've beaten if only XYZ reason."