Generally, in the US, if a letter is placed into the US mail system - the physical thing belongs to the recipient.
If concerned about physical copies being left with a therapist, then I would read it to them or use as a reference for what I wanted to say. OR show them on an ipad or laptop or something or just say before handing it to them - I would like you to read this but only if you assure me you will return it to me after reading.