。
She felt anxious whenever she thought about her future.
Literal
She [topic-は] her own future's matters [object-を] think [when-と] anxious was.
~のことを + verb of cognition is the standard frame for 'think/consider X,' where のこと softens 将来 into 'the matter of her future' — slightly more reflective than the bare 将来 + を. ~と is the temporal/conditional 'when, whenever' attaching to a verb in the dictionary form, marking automatic co-occurrence ('whenever she thinks, anxiety follows'). であった is the formal/literary past of だ — equivalent to だった but more written/elevated, fitting the introspective tone.