When that stone-hearted beloved refused to trust me in any way, my own faithfulness and loyalty thoroughly humiliated me.

The poet likens the beloved to a stone idol ('but') who remains cold and unmoved, completely refusing to trust the lover's devotion. The bitter irony here is that fidelity, which is usually a lover's greatest badge of honor, ends up becoming a source of deep shame and humiliation. Pledging your purest loyalty to someone who doesn't care at all turns that noble devotion into something that makes you feel foolish in your own eyes.