"One" is a song about a soldier who goes to war and is left severely wounded. The song's lyrics describe the soldier's pain and suffering, and they could be interpreted as a metaphor for the band's own struggles.
"Fade to Black" is another song that could be interpreted as autobiographical. The song is about a person who is feeling hopeless and depressed, and it could be seen as a reflection of the band's own struggles with mental health.
Of course, it is also possible that Metallica's lyrics are not about their personal lives at all. The band has never explicitly said what their songs are about, and it is ultimately up to the individual listener to decide what they mean.