Staying motivated with diet and exercise

Since being diagnosed with diabetes,  I have done alot of reading about this disease.   I know that diet and exercise are very important.  However,  I have trouble staying motivated and focused.   I work nightshift and often find myself tired and out of sync with others.  At work  people leave candy and snacks all over the place.   I rationalize in my mind that i can eat a blue berry muffin and cover it by taking more insulin.     I want to maintain a healthy life style, but like alot of people, I sometimes lack dicipline.