OK, I don't know what you said that was sarcastic and hurt her feelings, so I can't comment on that. But what's this about you "having her back?". Why does she need you to defend her - is she unable to defend herself? Can she not pay it back to someone who insults her?
Seems to me this may need some consideration on all fronts. Yes, you shouldn't have been sarcastic at her - especially given that she seems to be rather fragile. But do you want a fragile girlfriend? It just seems to me like you would be better suited to a girl who can fight her own battles.
What does the sentence in bold mean? It's quite possible it means she's been too busy to put much thought towards this. It could also mean that she's put it to the back of her mind and is letting her subconscious deal with the issue before she rushes into any decision. Kind of like "sleeping on it" but for days instead of overnight.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.