Quitting smoking.
My grandfather was a smoker. 2 packs a day of Kent no filter since he was 14 years old. He went to the doctor one day and the doctor told him that he had to stop smoking. He never smoked another cigarette after that visit, quit cold turkey.
It was too late, however, he already had emphysima (check spelling). He had to be on oxygen tanks for at least 3 times a day with tubes stuck up his nose. He finally decided that he didn't want to live on the oxygen everyday and stopped using it. He died 6 AM on Christmas Day that year. It ruined our Christmas that year. I was only 8 years old and did not get to know him as well as I wanted to. I can remember some things about him, but did not get to know him as well as I wish I could have if I were older.