In this excerpt from “porphyria’s lover” by robert browning, which lines suggest that the speaker doubts the depth of porphyria's love? and spread, o'er all, her yellow hair, murmuring how she loved me—she too weak, for all her heart's endeavour, to set its struggling passion free from pride, and vainer ties dissever, and give herself to me for ever. but passion sometimes would prevail, nor could to-night's gay feast restrain a sudden thought of one so pale for love of her, and all in vain: so, she was come through wind and rain.

In my opinion, those lines are: "Too weak, for all her heart's endeavour, To set its struggling passion free From pride, and vainer ties dissever." At this moment, Porphyria confesses her love for the speaker. But he is on the verge of doubting it because he thinks that she is too weak to give herself entirely to him, and because she is obviously unwilling to cut those "vainer ties". He doesn't really explain what these ties are and why they are vain. Maybe he has low self-confidence and therefore thinks that he is not worthy of her. Anyway, the following lines ("But passion...") function as a counterpoint because she actually gave up a jolly party and came in the stormy night to the cottage just to be with him - which probably means that she really cares about him. But it doesn't neutralize his initial doubt.