She was overwhelmed by the excitement of it all.

她为此兴奋不已

她兴奋 得 不知所措

of it all 不是词组。原句可以说成

She was overwhelmed by all the excitement of it .

三楼正解！在这里it all 代表使她兴奋的（前述的）一系列事件或过程或细节。