Yes, what you share is truly confidential, with a few exceptions:
If you share with your therapist that you are going to hurt yourself, that you are going to hurt someone else, if there is any child abuse, or if there is any elder abuse. These four things your therapist legally has to report.
A rare exception to confidentiality is if a therapist's records are subpoenaed by an attorney or a court of law. Your therapist will generally assert privilege on the client's behalf, though ultimately the final say goes to the court of law should they reject assertion of privilege.
If you choose to use your health insurance to help pay for psychotherapy, your therapist will submit a claim for reimbursement, which will include a diagnostic code. Additionally, per contract stipulations, insurance companies reserve the right to audit a client's chart (verbally or through submission of records) to ensure proper services are being delivered and further treatment is indicated.