Well, I don't have a lot of experience crawling across steel beams above heavy traffic, but I do have an anxiety disorder - and in my experience the primary characteristic of my thoughts when I'm panicky is that they're short, repetitive, and often not terribly useful.
Instead of making a neat list of the physical signs of stress, you could try to make your character's fear obvious in the way they describe those signs. Let the description heighten the anxiety - "My heart's pounding so hard I can hear it. It's so fast. Why won't it stop? It's making me feel sick. Stop, stop,
stop, I'm going to throw up,
stop it, why won't it stop?" You don't want to write a whole scene like that, but someone who's panicking badly isn't thinking rationally. You can give the reader a glimpse of that irrational thinking, then get on with the rest of the scene.
Often a sound or sudden movement will snap me out of thoughts like that, and you can use that to transition back to action. Your character hears something and jumps, freezes - then, since they're now paying more attention to their surroundings, they remember they're supposed to be doing something. They can take that first step onto the beam.
I'm usually not as aware of what I'm doing as I normally would be, like my body's on autopilot and I'm not really paying attention. I tend to notice an action after it's already happened; I won't think that I'm standing up, I'll just realize that I have stood up. You could describe this as an out of body experience, or with little gaps between actions. Your character is kneeling on the beam. Then they're halfway across. Then they're on the other side, with the sound of traffic behind them.
I don't generally have conversations with myself about how I have to be brave when I'm panicking.
Conversations about how mortifying it would be to throw up on the carpet, maybe.
You don't need to use all - or any! - of these ideas in every scene, of course. But I hope I've given you some ideas.