What happens if a neurotic person, with a difficult youth practices Zazen intensively, not doing psychotherapy?
Will his neurosis heal? Can he get to a deep understanding of zen, even staying neurotic?
I certainly insist on the fact that zen cannot and should not be taken as a therapy. So a neurotic person should see a psychotherapist. If he does not, it is hard to predict the outcome of an intensive practice of zazen. It can reinforce his neurotic ideas if , during zazen, he entertains his own thoughts; that can go to the point where he will really blow a fuse. Frankly, I would not recommend from my experience that a neurotic person practice zazen intensively, but to stay in control of its state instead, understanding the effects from his experience, and not exceeding his capacity of keeping some control of himself.
When the neurotic state is still there, but in control, I would think that there is nothing which should prevent any person from reaching a deeper understanding of zen. Both go together, as a deeper understanding of zen is actually a deeper understanding of oneself. If a neurotic person understands his neurotic state and succeeds in living with it, he understands himself. A deeper understanding of zen is principally that.