My problem with singing?

I think i have a nice voice when i sing with the song, as in i can match the tone rhythm and speed of the singing pieces of the song. But when i go to sing without music its as if ive never heard the song, my voice gets all rough and harsh and sometimes even cracks when i run out of breath and i have to hold a note. So i can sing well, but only with the music, any suggestions on how i can sing without needing the music? (By the way i do practice)