I believe this is about being in a relationship with someone who's too clingy/needy and emotionally dependent.

"I will perform a dead defying act And I won't fall I'm brave, I can save the very small I'm everything..."

He sees her as someone who can perform death defying acts without faltering. he sees her as a savior, as everything.

"but I'm nothing at all" But she's really none of that.

"You compare my light to the sky Why do you try, to make me better than the divine"

He idealizes her too much.

"It isn't the same, don't give it a name"

not sure.

Let me remain Let me remain

It's chocking her. He's too needy It doesn't leave a room for her.

"I'm everything I've ever been I'm everything I've ever seen I'm everything that slowly falls I'm everything and I'm nothing at all"

Who she truly is. She's just the same regular, mortal person she's ever been.

"I will perform a death defying miracle For someone with the chemicals to believe"

It's only the chemicals of love in his brain that make him so faithfully convinced in her ability to perform death defying miracles. Again, he's idealizing her too much.

"I am brave, but I've gravely understated I can't SAVE you from what you've taken and leave"

He relies on her to save him, to give him things he should readily have. But she can't. She can't save him from his troubles. Only he can make himself truly happy.

"You compare our love to the sun, what have you done?"

He's sensationalizes the relationship to an extreme, and it's irritating her 'what have you done?'

"There's a limit to the prizes we've won"

There's a limit what their love could provide.