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Hypothetical: I say negative stuff about John Doe to my psychologist, stuff which isn't defamation per se. My psychologist doesn't directly act on my statements, but does share my statements with third parties. Due to those statements those third parties cancel contracts with John Doe, stop buying products from John Doe, etc.

Could I use as a defense against defamation the fact that I thought my statements to John Doe wouldn't be shared with anyone?

Yes.

Making statements in a legally protected confidential context is not publishing them, and in most jurisdictions, defamation must be published to create a cause of action. In such a case the patient might well have a cause of action against the therapist for violation of patient confidentially, and a complaint to the relevant authority could get the therapist's license revoked, or perhaps a censure from the licensing authority, whatever it is the the jurisdiction.

Note "published" does not have to mean putting them in print, but does mean making them in such a way that general circulation of them is plausible.

No

Let’s assume that what you say to your psychologist is false and causes damage to John Doe.

Making the statement to the psychologist is the defamatory act.

Assuming the psychologist does not defame John, that is, she just reports that you said what you said then John has a case against you only.

You can sue your psychologist for breach of confidence, probably for the full amount John gets from you but her breach is not a defense for you.

Possibly

Qualified privilege is a defense in defamation. The statement would have to have been made without malice, be made in an appropriate situations and for a reasonable cause. If making the mistaken accusation, under assumptions of confidentiality, is reasonably related to the therapeutic goals of your sessions with the psychologist, it could be. The question is whether freely communicating the belief is critical to a reasonable purpose (such as getting your head straightened out).