How do I stop worrying about things at work I can't control?

You are learning the most freeing skill there is, telling what is up to you from what is not, and you are frustrated that you keep reaching for the things outside your grip anyway. Do not be too hard on yourself. Knowing the line and living on the right side of it are two different lessons, and the second takes practice. The grasping is a habit, and habits do not break the day you understand them. You will keep reaching for the outcome, the other person's choice, the thing already decided, because that reaching feels like doing something. But it only drains you, because no amount of worry moves what was never yours to move. So when you catch yourself grasping, name it plainly: this is not mine to control. Then turn your effort to the one thing next to it that is, your own next action, your own attitude. You will forget and grasp again. That is fine. Catching it, again and again, is the whole practice, and it slowly sets you free.