Hello, I am a psychiatrist.
I'm sorry that you are struggling with this on top of having lupus.
Do you feel like you have to apologize for having lupus? It sort of sounds like that.
You didn't sign up for it, right? Nobody does. You don't have to worry about letting people down when things are out of your control. Lupus is out of your control. If it was in your control, you wouldn't have it, because you'd be able to control it. You have it because you cannot control it. You had a flare up, because you cannot control it.
You didn't get lupus because you made a bad decision about something somewhere along the line or because you didn't eat right or because of anything else you could control. It struck you, because it just did
. That the way it is.
I suspect that you struggle with not wanting to let people down and/or not wanting them to have the chance to be disappointed in you. That said, you are a human being. Not a superhero, and you are doing the best you can with dealing with this awful disease that you certainly never wanted.
I suspect the answer here is for you to be gentle with yourself. You told your boss (after a year of not doing so), because you needed to tell her. Now she knows, she was receptive, and it's okay.
Does this make sense?