(Probably won't see this but putting my thoughts out there:) You do not need to apologize. You have fought so hard through so much. 
When I read your blogs about your health and struggles, I was heartbroken 
(I actually cried alot, not because you would be stopping D, but because you had been through so much and pushed so hard. (Though of course I will also miss your singing and songs, but what matters is your wellbeing.)) From your blog posts you seem like an extremely caring and kind person. You deserve to be able to rest and take care of yourself, and to be happy. ((I hope other fans feel the same!))
As the date of the hiatus approaches, I often sit and hope for your wellness and happiness. I will still be listening to the songs you have already made (you made so many!) and randomly singing them in my house. Your music is very magical and expressive and helps me alot mentally during hard times. 
I don't think anyone can ever top your talents.