This we have now

This we have now
is not imagination.

This is not
grief or joy.

Not a judging state,
or an elation,
or sadness.

Those come
and go.

This is the presence
that doesn't.

- Rumi, from The Essential Rumi, trans. Coleman Barks, p. 261

That's one of the few poems I've been able to memorize. I repeat it to myself when I'm starting to freak out about something. Try it yourself, it works wonders!