Don't start with a theme or a topic, with the idea that you are going to write about something. That way you're likely to write a piece that explains or describes something, rather than embodying it.
Don't start out with some idea of being "poetic", e.g. with the thought, "the Romantic Poets wrote about skylarks, nightingales and the beauty of nature so that's what a poem is and I'll do the same thing." That way you're likely to write a dead poem, a simulacrum, a copy.
Don't start out thinking that expressing emotion is enough. That way you'll end up with notes for the therapist. The idea is to embody emotion, not state it.
Try starting out with an image, something that is somehow important to you, something that recurs in your consciousness. Then explore the image. Let the poem or piece of prose write itself. Try to find out what other images that image is connected to and why (but don't worry about explaining why).
Try starting out with a sound, a sentence, a phrase that someone uttered and that has always stayed in your mind. Then let the phrase expand, let the poem or piece write itself ...
Try writing "rubbish" if need be, until by chance you hit on something that excites you. It may be just below the surface of your mind, stay with it and let it rise to the surface. You may not know in advance what manner of fish you're catching. It will reveal itself.
Try concentrating very hard on the shape your feeling is taking as you try to get it down, the specificity of the situation, what things look like, taste like, sound like.
In other words, try forgetting yourself. Just concentrate on getting down as precisely as possible what's happening in the poem or the piece of prose.