Presumably you have a line manager. Can you not raise these issues with him/her - as your assistant is also experiencing difficulties, perhaps you could both talk to the manager? Concentrate on issues that are making your role difficult but it's probably better not to identify specific obstructive members of staff.
After 30 years of service, I don't think you should resign. Get signed off by your doctor and hopefully the break - along with some support from management to deal with the difficulties you are encountering - will enable you to continue.
If, after your break, you feel the job is still just too stressful, start looking for something else or, if you can afford it, just leave.