I'm surprised your therapist is suggesting disclosure as a way of dealing with the demands of your employer.

No, I would not disclose. I don't see any reason an employer needs to know about my psychological issues. I can't imagine my manger telling me about her own psychological issues or emotional struggles.

It would be much better to have a discussion with your employer to make sure you are both on the same page about what is expected of you. They may think that they are giving you a great opportunity for more hours, if they think that's what you want. I'm sure that some part-time workers would welcome that, but that's not what you want. What the company wants and needs is important too, so talking together would really help clarify both what you want and need (all you need tell them is you have other commitments. That your other commitments are not working too many hours is not something you have to specify ), and what the company wants and needs.