on (one's) lonesome
Completely alone. Primarily heard in UK. My friends have all gone on holiday and left me here on my lonesome. You can't leave a boy that young on his lonesome! He needs to be supervised all the time!
all by (one's) lonesome
Completely alone. My roommate left town for the weekend, so I'm here all by my lonesome.
all by one's lonesome
Rur. all alone; by oneself. Mary's folks went out and left her all by her lonesome.