Murmuring how she loved me (21)
This could be really sweet. After all, Porphyria is "murmuring" softly in the speaker's ear. How romantic! Or it could be that she's still hesitant, and can't bring herself to say it more directly.
To set its struggling passion free From pride […] (23-24)
The speaker says that Porphyria's "passion" is "struggling" in her heart. She's too "weak" to set it free. Love is getting figured in terms of freedom and restraint again – Porphyria is constrained by something (the "vainer ties" that never get explained) and can't love freely.
A sudden thought of one so pale For love of her, and all in vain (28-29)
The speaker imagines that Porphyria came to his house that night because she couldn't bear the thought of him suffering alone in his love for her.