Alone (Poe)
common spring
Spring as source, not season—his emotions don't come from the same wellspring as other people's. He's claiming fundamental difference, not just moodiness.
I lov'd alone
Double meaning: loved in solitude, and loved things nobody else loved. The italics emphasize both the 'I' and 'alone'—he's underlining his isolation.
Then — in my childhood
Pivot point. He's been describing effects, now he'll explain the cause—something that happened in childhood that 'binds' him still.
good and ill
The mystery came from both extremes—beauty and terror, fountain and torrent. His vision sees darkness in everything, even golden sunlight.
When the rest of Heaven was blue
Key detail: he sees the demon-cloud while everyone else sees clear sky. The parenthetical makes it worse—he knows his vision is singular.