Focusing our attention on physical sensations helps to relieve stress by shifting our attention to the present moment.
By simply being aware of this moment (which is all there is) we naturally decrease our worries about the future and our sadness/regret/rumination of the past.
Don't try too hard
We don’t succeed at meditation by trying hard. Instead, we learn to use a gentle, easy focus. By not trying too hard, we invite a new way of relating to ourselves. After all, we are human beings, not human doings!