You don't need to tell your mom that you are in therapy. It is a private and personal thing that you are doing for yourself. While it is always nice to have a parent's approval even when your a grown adult, when that approval is absent then you have to just do what is right for you. If your not comfortable discussing the topic, just don't bring it up. You don't have to lie or hide it, just don't mention. If your asked point blank, you can say that this is something you are doing for your own well being and you wouldn't be doing it if you didn't think it was helping. It is helping and that is the end of it. No further explainations are needed.