When does the personal 'I' in poetry cross the line from sharing universal experience to indulging in confessional therapy?

It depends upon the subject but also on the implications of the use of the word 'I'. Most inexperienced poets (by that I mean poets who think they can be poets without ever really reading poetry) misunderstand the use of this particular personal pronoun and assume that it represents the poet. It needn't. In fact, in my opinion, it shouldn't. The poetic voice is a similar beast to the narrative voice in fiction. All poetry is artificial in that it is not language as spoken but written. As soon as you sit down and start thinking about what you are going to say you assume a persona, there is a moral and aesthetic difference implied between the poet and the poetic voice. To abuse this by merely 'saying what one wants to say' is a great blight on modern poetry. It becomes about expression of the self rather than the wider, and more noble, goal of expression of the language.

There aren't that many people whose lives I am interested in (if there were I would watch Jeremy Kyle). But I like to hear the way different people use language.

One of my favourite poets is Sylvia Plath, which would seem to be antithetical to everything I have written above, but she used language to obscure with metaphor the emotions she wrote about. This is a skill that goes way beyond the disparaging soubriquet of 'confessional poet' which has been conferred upon her. And the 'I' in her poems is a shape-shifting polymorphic entity, a series of hats worn, a fiction.

Good question Cynthia. It's got me fired up.

I use it as a viewpoint character giving his narrative, and only bits of the poem actually reflect me. For example, I don't really have a dead wife rotting away in an attic, but some of my feelings about stuff are hidden away in a poem that talks about that in the first person. Besides, you write poems partly to exorcise stuff that bothers you - you sure as hell don't do it for the money, and you've got to get something from it (enjoying creating little bits of art is probably the main thing you get out of it though - that passes the time).

You can claim that to us, DG. But it won't stand up in a court of law.

Encore une fois il a raison DG ; )

je suis d'accord, si c'est pas interdit d'utiliser le mot "je"?

OK, it is an interesting question, though I dont' recognise this crossing of some kind of line. Unless, I suppose, that when the "I" ceases to be oneself, and becomes the persona adopted (wittingly or not) for the purposes of the poem, then it crosses the line from purely personal to unversal?
I do think that it crosses in precisely that direction: from the "I means me" to the universal first person pronoun; as the writer gains in maturity and understanding. Or summat like that.

Bonne, comme Julien dissait.

Plus: there is nothing wrong with putting a poem that you have written as therapy into the public domain (wait for it) as long as it has levels that work for other people. If the poem is JUST therapy and nothing more, then - sure - the rest of us aren't interested. But if we're likely to find your personal tragedies and your suicidal tendencies very amusing then we want to hear them (we all like a good laugh). Failing the foregoing, if it's written in a way that is thought provoking or actually poignant for anyone who has not had your life experience (and therefore isn't bothered by tales of your specifics), or if it is heavily disguised under a seemingly just plain interesting narrative, then put it out there.

My point here is that you can put out poems you've written as therapy, as long as you've alos looked outside yourself and tried to put stuff in that will engage or entertain other people.