I need help explaining to work why it’s so important to stick to my set shift pattern.

So I work 30 hours a week as a staff nurse on a cardiac surgical ward. I absolutely love my job but it can be very mentally and physically draining. Occupational health advised my ward manager that I should work a set shift pattern. Now due to staff shortages and such a busy work load work do not always stick to this shift pattern. When I’m working my set shift pattern I feel I’m able cope with work life as well as being  a mother to 4 kids, two of which also have autism. When work do not follow my set shift pattern I can cope at work all be it im very tired and can get overwhelmed however  everything at home becomes to much to bear. 

I have tired explaining to work the importance of following my shift pattern but I can never seem to find the words face to face . I just get told well patients needs come first or that the staffing won’t allow my shifts, or to book annual leave to allow for my shifts and I back down. However I’m one week into the next 6 weeks of work not following my set shift and I can’t cope. I’ve already had a break down. I find my daughters autism to much to deal with as I’m too exhausted  from work. 

The reasons I asked for set shifts is because nursing is such a demanding  job. My shift pattern allows me to have sufficient down time between shift. As I also have sensory processing issues. I also like knowing my shifts in advance as this settles my anxiety.  What I struggle to explain is why my autism effects me so much emotionally maybe it’s because I don’t understand myself. Having a set pattern also helps ease my daughter anxiety about when I’m not home. 

So my question are do work have to follow my shift pattern regardless off patient need and staffing? 

How can I explain why I need these set shifts. As I’m sure they doubt I have autism as I cope(mask) so well at work. 

What can I do if work insist on not sticking to my set shifts. 

All help would be very much appreciated I don’t want to leave my job but I just can’t cope with them not sticking to my soft pattern.