"Doubt"
Being human, Anya quickly learns, comes with many doubts. She's painfully aware of how awkward she is, and while she hates humans for their complicated and confusing rules, she yearns for their acceptance nonetheless.
Then there's sweet, understanding Xander, who loves her despite her flaws. She worries, sometimes, that she's not good enough for him, but then she looks at the ring on her finger and her doubts disappear.
Nine months after first putting on that ring, she's alone in their apartment, wearing a wedding dress stained with blood and wondering how she could have fallen so far into denial.
"The Power of Fear"
It's not until later that night, when he's alone in his motel room, still wearing his tux, that Xander realizes how badly he's fucked up.
He became beset by fear and doubt after Buffy's funeral, when he realized their lives - their engagement - would continue. But instead of being a man and speaking up, he tried to deny them and feign happiness.
The memory of Anya standing there, an angel in resplendent white, haunts him. He didn't even tell her how beautiful she looked before breaking her heart. Fear made him deny his love for her; now he's lost her forever.