How should one interpret ～ていた + だなんて

Question

I came across the following sentence in volume 12 of the manga たいようのいえ and was unsure how to interpret the expression at the end. Could someone please explain how the "だなんて" changes the meaning or tone?

私・・・こんなに大切なものを奪{うば}ってしまっていただなんて・・！

I...I stole such an important thing!

Context: The speaker previously took credit for something she didn't do and is now regretting it.

Also, while I'm at it, could someone please describe the function of こんなに? Does it function as an adjective and modify もの, adverbially and describe the manner in which she stole, or is it more of an interjection (or none of the above)?

Answer 1

「Mini-Sentence + （だ） + なんて」

is a common and informal way of saying:

"I never thought that ~~~~!"

This structure is used to express one's sense of shock, surprise or suspicion about something. Please never use it in formal speech. I will say 'please' one more time.

「[私]{わたし}・・・こんなに[大切]{たいせつ}なものを[奪]{うば}ってしまっていただなんて・・！」

My own TL of that would be:

"I never realized what an important thing I had robbed (someone) of!" or

"I never thought I had robbed (someone) of such an important thing!"

「こんなに」 adverbially modifies 「大切な」, and it does not modify 「もの」. The key word is 「に」 here. 「～～に」 functions adverbially.