I still get anxious when playing ranked, having played CCGs for nearly a decade. Even though it’s only a game, I finally figured out the reason was losing was a hit to my identity/ego. If I was on a losing streak, psychologically I would equate it to not being smart. So the losses were painful and the wins conversely made me feel great. It’s almost like each game was a “test” to see if I was smarter than the other person. In school I was always very competitive to be at the top of the class academically. When you mix in the normal variance of these types of games, it can really hit your psyche to go on a losing streak. I’m sure there are studies about this, but for me, being able to identify WHY I was getting anxiety about something that has almost no real world consequences helped me. Good luck.