After a long year of exercising, why have i not seen any results?

It has been nearly a year since i have been trying to get a 6-pack and an all around good body. I have been gradually increasing my maximal exercise to see better results but nothing is happening. I am roughly 6 feet tall and weight about 171 pounds. Since i have not been seeing any results, particularly my abs region, i have been highly upset with myself. I focus solely on sit-ups to get these abs. On a side note, however, i do eat McDonald almost every other day, particularly a double cheese bugger. Should i suppress my carvings for McDonald for a healthier diet?