'Tis dark: quick pattereth the flaw-blown sleet: "This is no dream, my bride, my Madeline!" 'Tis dark: the iced gusts still rave and beat: "No dream, alas! alas! and woe is mine! Porphyro will leave me here to fade and pine.— Cruel! what traitor could thee hither bring? I curse not for my heart is lost in thine, Though thou forsakes a deceived thing;— A dove forlorn and lost with sick unpruned wing."
Even though we were just told that Porphyro and Madeline have both "blended" into Madeline's dream, Porphyro announces to her that it's not a dream. The repeated mentions of the storm outside make you think that this sounds like the real world. Then Madeline talks about how her heart is "lost" in Porphyro's, though, making it seem like they're still immersed in her dream. This is almost as frustrating as the ending of Inception.
Contrary to Porphyro's expectation, Madeline is not happy about this news.
The funny part is that she's unhappy for the reasons you would have expected her to be if she'd been told that it was a dream: she's upset because Porphyro's going to leave her and she'll be left to "fade." Normally, you'd think of a dream as the thing that leaves you high and dry—you enjoy it for a little while, and then it's gone and you have to go back to the waking world where you spend the vast majority of your time. For Madeline, though, the dream offers constancy and vibrancy.
Madeline ends by talking about how she's been "deceived." She uses a metaphor to compare herself to a sad, sick dove that's lost and alone. This is kind of cool, because instead of thinking she's been tricked by her dream or her imagination, Madeline appears to be cheesed off because she's being faked out by reality.