Depression after treatment is not unusual.  During treatment, the patient is busy doing specific activities to fight cancer, but when that ends, anxiety can increase.  Also the permanence of the losses begins to sink in.  If extra rest and cheerful activities don't lift your mood, talk to your doctor about the possibility of an anti-depressant medicine.  A counselor experienced in working with cancer patients could help you explore other coping strategies.  The physical changes from cancer treatment added to the emotional stress can be a very real cause of depression.  I'm sure you've learned many coping skills and how to deal with the medical community during your forty years coping with MS.  Let your doctor know you need some extra help.