。
She felt anxious about her future.
Literal
She [topic-は] her own future [toward-に] anxiety [object-を] felt.
~に不安を感じる ('to feel anxiety toward X / about X') uses に to mark the target of the emotional reaction — a productive structure for emotions directed at a stimulus (恐怖を感じる 'feel fear at,' 違和感を覚える 'feel discomfort at'). 不安 ('anxiety, unease') is built from the negation prefix 不- + 安 ('peace, ease') — literally 'un-ease.' Slightly more direct than 不安だ ('be anxious') because it foregrounds the act of feeling.