Before playing, many people will sart thinking about what and how they play after they took a breath. That's like an olympic diver thinking about his dive after jumping on the board! If you watch them on tv, you'll see that they first visualize their jumps and then, once in the air, they let their body do the movement that they rehearsed thousands of times. They would for sure end up making a huge splash if they sarted thinking about it up in the air.
It is important that you know what you'll be doing and how BEFORE you breath. You can then play more confidently and you'll make a better start.
So you need to 1) vizualize, 2) take a good breath, 3) let go and play
Don't start thinking after you breath!
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.