I would be the best boyfriend I possibly could to my GF for a few months to make her as happy as possible before I tell her the news.
Stay with my GF and my family for as much as I can, make every minute count.
On my last day, I would just be very nervous but accept my fate, never leaving my family's and GF's sight, gonna be with them with my last few minutes on earth.
(I'm not a violent person like whatsoever, so I wouldn't do any crazy stuff before I die)