How can this help?
When you are in a slumped position, your ability to breathe deeply is impacted.
Sitting up straight and tall could make it easier for you to take deep, slow breaths. This then allows your body to take the oxygen it needs from the air you breathe in, reducing the ‘alarm’ state caused by insufficient oxygen.
Dropping your shoulders also helps you to feel more relaxed, as it soothes tense muscles.
Find a short phrase which helps you remember this technique, like "head over heart, heart over pelvis" or "shoulders down, head high".
Then, when you're feeling low, remind yourself of this technique and give it a go.