Obviously I could be wrong - the first verse really makes it sound relationship-ish. But this could still be a parallel still to his mother. This third verse could easily be about birth:
"On the sheet I see your horizon"
Born in a hospital bed
"All of me pressed onto you"
Being held by mother
"But in this light you look like Poseidon"
Feeling small next to her, as a child to an adult
"I’m just a ghost you walk right through"
She left because he was so insignificant
Obviously I could be wrong - the first verse really makes it sound relationship-ish. But this could still be a parallel still to his mother. This third verse could easily be about birth:
"On the sheet I see your horizon" Born in a hospital bed
"All of me pressed onto you" Being held by mother
"But in this light you look like Poseidon" Feeling small next to her, as a child to an adult
"I’m just a ghost you walk right through" She left because he was so insignificant