Besides the mechanical/knowledge parts of it, one important thing (at least in my opinion) is cultivating an improvement focused mindset. Playing To Win [0] is a good overview. If something in the game doesn't go your way, blame your own understanding of the game and try to figure out what you didn't know or what you did wrong.
Team-based games like League of Legends can be kind of frustrating simply because your control over the outcome of the match is proportionally less than in a 1v1 game. It doesn't mean you can't influence the match to an outsized degree, but even the best players lose matches sometimes because their teammates perform too poorly to be "carried" to victory. Managing this frustration and taking a long view of performance is key to improving at these kinds of games (or at least climbing the ranked ladder).
Team-based games like League of Legends can be kind of frustrating simply because your control over the outcome of the match is proportionally less than in a 1v1 game. It doesn't mean you can't influence the match to an outsized degree, but even the best players lose matches sometimes because their teammates perform too poorly to be "carried" to victory. Managing this frustration and taking a long view of performance is key to improving at these kinds of games (or at least climbing the ranked ladder).
[0]: https://www.sirlin.net/articles/playing-to-win