As suggested be sure to ask your therapist what they have to divulge by law before revealing anything and take the answer into consideration.
My advice though might be contrary to what you want to hear. I think you need to explore why you are smoking, when you are smoking, how much you are smoking and if it is filling the need you are using it for.
All the above is best discussed with your therapist. So even at the risk of having your parents find out (and they might be surprisingly supportive) so I suggest you do bring it up in therapy.
For many users pot relieves pain and other symptoms of illness, both mental and physical. However, if it is used as a bandaid or for a mental escape it will delay your healing and, excuse the expression, castrate the therapy. On the other hand, if it is helpful you may even garner a prescription for it.
Keep us updated on what you decide and just remember that you are worth what ever it takes to heal.