Help with resignation letter

Hi all, I am in need of help. I am looking to write a resignation letter but not sure what to say, given the situation. I apologize in advance for the long post. 

Background : I have worked at a small geripsych facility for a little over 2 years. I got the job as floor nurse as a favor from a former manager who is now my current manager. At the beginning of this year, I accepted an ADON position due to some restructuring/maternity leave issues. I took a pay cut for this position because at the time, I felt like I was repaying the favor to my manager. 

Since taking this position, I've been very unhappy. It has made me more anxious and actually dread going to work. Even on off days, I hate looking at my phone because more often than not, I am being asked to come in and cover the floor or help with admits. Yesterday, I wasn't even asked - I was told I have to come in to work a night shift. All of this without any bonus or any overtime. Meanwhile, our floor nurses have a great bonus structure with pickup and monthly bonuses. I realize that this is what I signed up for, but the lack of compassion or respect for my home life has really pushed me over the edge. I am done, mentally, and have been for sometime. So I am finally ready to write my resignation letter.

So, to get to my question. I would be willing to demote myself and go back to the floor full time. I am aware this would be provided there is a position, which there most definitely will be (I am not the only one that feels this way about management). But but I am not prepared to fully leave my employer, though I guess I should be. I am finding it hard to word my letter that makes it clear that I want to step down but stay as floor nurse. 

Does anybody have any experience with this?