You had games where you didn't care at all. And you popped off. But why do we play better when we are relaxed? Because just knowing that doesn't help you fix it.
Here is what's actually happening in your brain. When the game matters to you, you think on what you are doing, and you try to perform, and that makes you focus on your mechanics instead of the game, and that kills flow state. So the more you need the win, the more that part activates. The more it activates, the more interference you have. The more interference you have, the worse you play. And the worse you play, the more you need the win. That cycle is called the law of reverse effort. The harder you try to play well, the harder the game becomes. You cannot fix this by caring less. You cannot just tell yourself to relax. Your brain knows better than that. What you can fix is what you focus on. You need to focus only on your team and on what the round needs you to do. And when your attention is not on yourself, your brain stops watching itself. Self 1 trusts self 2. Your mechanics run on automatic. You stop playing scared and start just playing.
Ask any questions!