And this brought forth a dream and soon enough
This dream itself had all my thought and love. (2.15-16)
"Love" is another way for Yeats to express truth. See, Yeats was pretty given over to mysticism, especially in his later years, and mysticism is all about the connection between love and truth. At this moment, though, he's exploring the ways love can lead a poet away from more central human truths, because a fantasy world is often more alluring.
Heart-mysteries there, (2.18)
What's a heart-mystery? Let's see. We know that, for Yeats, "truth" wasn't factual truth. His poetry is far more concerned with emotional truths, and maybe that's what he is referring to when he says "heart-mysteries." These mysteries are the problems that require the most work because they are messy (and they're messy because they're true).
It was the dream itself enchanted me: (2.20)
It takes a certain kind of dedication to admit your own failings… especially when those failings were delightful illusions. Notice how Yeats still uses the language of mysticism when discussing poetry? Poetry can "enchant." Not bad work, eh?