It's easy to hear the bad thoughts after that. It's like she can't help them. Elsa's having a child soon. The last thing she needs a stupid kid mucking up her life.
Anna's so selfish, putting this on Elsa now.
But god she's so lonely. Elsa alleviates that, makes her forget all the shit. Simplifies her life and makes it... whole.
Her heart aches, feels like it's dissolving because Elsa gives her so much, so how can she add any value to Elsa's life?
She can't.
No wonder Elsa said no.