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She was at a loss for words to express her feelings.
Literal
She [topic-は] her own feelings [object-を] express words [at-に] was-perplexed.
言い表す (言う 'say' + 表す 'express, represent') is a compound verb meaning 'to express, articulate, put into words.' 自分の気持ちを言い表す言葉 ('words to express her feelings') uses the dictionary form 言い表す attributively to modify 言葉 ('words') — a relative clause sitting before its head noun. 当惑する ('to be perplexed, at a loss, bewildered') with に marks the cause of perplexity. A recognizable moment: emotion outpacing the words available for it.