Even your provider can write that you "are being medically treated" but not what for and scheduling accommodations are needed as to not interfere with treatment; it does not have to be disclosed and you can ask them not to disclose but yet if you do want this back-up to enforce why you cannot work more hours so that you are covered with the employer.

There are some employers who will take advantage of you no matter what and sounds like this is one. I understand the delicate line walking though because you want to keep the job and be accommodating, but yet at what cost; I would too. I can see where the documentation might help you to set boundaries but I still can't see where disclosure will help, especially knowing this is a boss who lies is a tribute to his character; questions for me, that should you disclose, how he/she will use your medical information in the future and cannot be trusted with it. I would not disclose to someone like this.

If they are so short-handed, are they short staffed? Have they considered hiring someone part-time or as a floater to pick-up shifts? No need to answer, just something wanted to throw out there that might want to mention if it had not been considered and to possibly discuss the next time you talk to him/her about scheduling, also if comfortable doing. This might help take the pressure off.

I can understand your dilemma and hear your frustration. We have to do what is best for us and it sounds like you are taking the steps to do just that. Do what you need to to take care of you for your protection and to be well. Take good care meanwhile.