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?
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)?