I feel like the best way to avoid getting upset or 'salty' at losses, and this is kinda just how I've been looking at it, is to enjoy the full breadth of the game rather than just the parts you directly control.
If you can appreciate how cool the things other players are doing, even when it's your opponent, then instead of being upset that you lost you can express awe at the skill your opponents demonstrated at utilizing the tools they have access to or just finding fascination with the tools themselves.
I'm keeping this kinda general because for me this applies to both Magic the Gathering and Fighting Games, though I imagine this can apply to any competative multiplayer experience, or even potentially cooperative experiences such as Tabletop RPGs.
This also just comes down to a little bit of rejecting pure individualism and understanding that in every competitive situation there will ALWAYS be a losing party, and it's fine if you are often amongst that party, as long as you don't attribute your losses as a meaningful failure of your own character.
Though this also comes with my own caveat that I enjoy these things as hobbies, and have never seriously considered competing for money or as a career, and understandably if you live within the system of capitalism, pursuing competition as a means of livelihood can come with it's own set of stresses. That's it's own discussion altogether.