I actually found it myself. I remembered the collection I had read it in and searched through it. It is _The Secret_ by Denise Levertov.
_Two girls discover_
_the secret of life_
_in a sudden line of_
_poetry._
_I who don’t know the_
_secret wrote_
_the line. They_
_told me_
_(through a third person)_
_they had found it_
_but not what it was_
_not even_
_what line it was. No doubt_
_by now, more than a week_
_later, they have forgotten_
_the secret,_
_the line, the name of_
_the poem. I love them_
_for finding what_
_I can’t find,_
_and for loving me_
_for the line I wrote,_
_and for forgetting it_
_so that_
_a thousand times, till death_
_finds them, they may_
_discover it again, in other_
_lines_
_in other_
_happenings. And for_
_wanting to know it,_
_for_
_assuming there is_
_such a secret, yes,_
_for that_
_most of all._
I realise now that I had the description a little bit wrong. That's the trouble with memory I suppose.