I will just add that there is nothing at all which prevents someone who will benefit from seeking help from a qualified mental health professional for such issues. In fact, I would encourage so if someone would benefit.
Therapy and, if the doctor approves, Zazen can go hand-in-hand.
One of the wonders of Zazen is that it lets us sit with anxiety when anxious. For example, many times someone may become depressed or anxious, and then become even more depressed and anxious about being depressed and anxious, doubling their power! Zazen lets us treat the passing drama of the mind as just temporary, passing weather, and not fall into taking our mood so seriously. The sadness and anxiety may still be present, but we just consider them objects in the room like the table in the corner, nothing to resist, think about or be pulled into. Suddenly, some of their extra fuel is gone.
However, if there are underlying issues that are the root cause of the depression or anxiety, Zazen may not be the most effective therapy alone, and one should talk to a doctor or mental health professional and deal with it. (I like to say that, if one has a bad tooth, one doesn't expect Zazen alone to deal with it: One goes to the dentist.) If the doctor approves, one can keep sitting Zazen too.
Sorry to run long.
Gassho, Jundo
STLah