Take deeper, slower breaths.
They can be done anywhere. They don't have to be huge. Or in any way noticeable.
When you exhale, imagine that you are whoooooooshing out, blowing out all your worries.
And imagine that you are drawing in happiness. Hope. Beautiful things to come. Beautiful, sparkling mountain air. Lovely, alpine meadows. Fresh. Crisp. Sparkling. Pure. Full of Spring sunlight. Flowers everywhere. The sight of them. The scent of them. Near you, birds are singing. And a mountain stream is dancing. Merrily. Full of joy. Your heart lifts. You take off your shoes. Dip a toe in the cool, refreshing water. Then walk barefoot on the soft, green grass...
Or think of any lovely, natural landscape you like. A green park. A river. A lake. An ocean. Picture yourself there. For a moment's vacation. While gently breathing in... just a little deeper... a little slower...
Until you start feeling calmer... more relaxed...