Have no doubt, I am focused, but I have learned to trust my practice. I have accepted that I cannot do reliably any more than what I have practiced (or what my practice has prepared me for) and that many shots I make are more luck than skill. And you are 100% right, you are likely to perform differently in a new environment, a lot comes down to adaptation. If the score works in our favor, we largely are satisfied, if the score doesn't, we complain or get tense. But on the whole, the only thing you can do is learn and adapt at best based on your practice. Beyond that, consciously trying to do better will interfere with what you have trained given the speed of the game. So there is a level of acceptance that comes with understanding as the wise one said, "Do or do not - there is no try." You just do and let yourself adapt or call into mind the goal state and let the points reinforce it over time. Hopefully you adapt quickly enough to get the win, but sometimes you don't. The only thing that is unclear is if complaining makes you adapt faster - that is a personal empirical question to which my answer is no for myself, but I think it could be different for someone else based on their experiences. But for me, even if someone was to show I was wrong and that if I worked on trying harder, I would play better, I decided that the stress was not worth it. I remember when I was borderline 1950 trying to break USATT 2000. I complained for a long time about it to my coach and my coach felt I was already playing at that level, I just lacked confidence in my play. So what did I do? I started driving to play singles leagues on a weekly basis, sometimes once a week, sometimes twice a week. After I broke 2000 in the leagues, I broke 2000 in tournaments a few months later. After breaking it in leagues in fact, I relaxed and saw that it was more or less inevitable that I would break it as long as I continued to play tournaments.
As you can see from my signature, this basic reinforcement via relevant experience approach is basically how I recommend one should approach anything. And don't stress out, try to do it in low stakes environments as much as possible, so that when the high stakes becomes the reality, you are ready.