So often when people sit down to write poetry, they feel that the act warrants an outpouring of feelings. The problem is that feelings are universal, and if you write something like, 'I feel sad', while I'm sure your readers can empathize, they will also be bored. You do not want to tell your feelings when you write a poem, what you want instead is to create images in your reader's mind that evoke a strong emotional response. Poetry is the language of imagery.
You may have heard the phrase 'Show it, don't tell it.' This is a good rule of thumb for writing in general, but especially for poetry. Poems have to say more, they have to cover more emotional ground in a few lines than a regular story. Poems are like the concentrated version of literature. If you find yourself having to explain what you mean to the reader, either within the poem or after reading it aloud, then you should go back and revise. Poems don't have to mean the same thing to everyone. Though the images you create should draw upon some universal truths in the human experience, everyone's reaction to a set of words or pictures will be different, based upon their unique perspective. So never argue your meaning with a reader, don't seek to explain your poem. If something requires clarity, go back to the work itself and make it clear.
Archibald Macleish had this to say on the nature of poetry:
A poem should be palpable and mute
As a globed fruit,
Dumb
As old medallions to the thumb,
Silent as the sleeve-worn stone
Of casement ledges where the moss has grown -
A poem should be wordless
As the flight of birds.
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A poem should be motionless in time
As the moon climbs,
Leaving, as the moon releases
Twig by twig the night-entangled trees,
Leaving, as the moon behind the winter leaves,
Memory by memory the mind -
A poem should be motionless in time
As the moon climbs.
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A poem should be equal to
Not true.
For all the history of grief
An empty doorway and a maple leaf.
For love
The leaning grasses and two lights above the sea -
A poem should not mean
But be.
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This poem distills the idea of show it, don't tell it; the imagery he uses evokes the emotion like a clear, high note.
When you are writing poetry it flows from a deeper source than your regular, informative inner voice; I will call it the 'thinking voice'. Sometimes it is hard to get to that place where the poem first pours out, it's almost a subconscious level where the barest and rawest of images begin in your memory and grow out into new and fantastic branches of thought to explore. We are closest to that state when we are between sleeping and waking. What's difficult is to remain in that twilight world where all the words are new and strange, and record them without going back into your conscious, everyday thinking voice. If you reach that point, just write and write, keep putting the words down. If you once go back and start to edit or reread, your words become rehearsed and you'll find you've clicked back over into your thinking mind again. The poetry mind is more a state of feeling and experiencing as you write, the thinking mind is much more clinical, and it will sound that way on a page.
Once you have laid all the words on paper, then you can go back and shape and organize your thoughts. You may find 3 or 4 poems are lurking in the material you've captured. Don't over edit your words, or they lose their visceral connection to the original ideas.