Do NOT do breathing exercises to curb anxiety, because that will just reinforce the pathways in your brain and your anxiety will keep recurring.
If you're noticing you're mind is dwelling on anxiety-inducing thoughts you need to refocus on your current task.
Just keep doing that whenever symptoms popup.
The symptoms will popup less and less frequently, and you'll have developed such strong coping mechanisms that you automatically handle the issue.
This is approach is called "cognitive behavioral therapy" or CBT, and is an evidence-based approach used widely. You can consult a therapist and get taught CBT skills by a professional. (That's what I did)
In addition to learning CBT techniques, and get regular exercise and enough sleep. I recommend you cut out all caffeine until you have developed strong CBT skills. Of course, rule out any underlying medical issues and work on any medical conditions (obesity, bad posture).
Once you're comfortable you have anxiety under control you can slowly re-introduce caffeine.
Source: I used to have severe and debilitating chronic hyperventilation and anxiety, but sought help from a CBT therapist a few years ago. Now when I am living my life and notice I've started hyperventilating (or dwelling on an anxious thought), I can immediately stop it by refocusing. It's changed my life and I highly recommend learning the CBT techniques.
I highly recommend a few sessions with a CBT therapist. If you're not in a position to do that you might be able to learn about CBT techniques from elsewhere, such as YouTube.