Anxiety, stress and burnout
We are leading very different lives to when mankind first started dominating the planet. However despite us changing in many ways, our basic survival mechanism is the same. We become aware of a danger and our brain responds to keep us alive. The difficulty is that today our dangers aren't a simple run from the tiger or fight for your food but any combination of emotional, physical or perceived stresses (a stress is any external pressure on any of our systems).
What should keep our response in line with the actual situation is our frontal cortex of the brain, it will send the 'it's only a truck rumbling past not an earthquake' message so that we can remain calm. However the more time you spend in the stress response part of the brain the weaker the connection gets and the harder it is for you to remain calm. This mismatch between the actual event and how we respond is what leads us to feel unwell. Basically you end up breathing, pumping your heart and trying to oxygenate yourself so that you can run, but, you are just sitting there.
It can get to a point where as this is happening so much the muscles learn to keep behaving in this way and will then in turn keep signaling the brain that you are in danger, even when you are relaxed. This leads to anxiety as your brain tries to make sense of the signals.
Our breath is what hacks the nervous system and can tell the brain we are safe, it stimulate the Vagus nerve to calm our mind and body.
The breath you need is in line with what you are doing and focused on the exhale.
Burnout is the end extreme of this where the body hits exhaustion as it can't keep working at that pace. You get run down, sick, exhausted and even exercise can make you feel worse.