、。
She said goodbye with a smile, but inside she was in terrible pain.
Literal
She [topic-は] smiling-face [with-で] goodbye [quotative-と] said [but-が], heart [possessive-の] inside [topic-は] very painful was.
The contrast between 笑顔 (えがお, 'smiling face') and 心の中 ('inside her heart') creates a powerful emotional gap — her exterior and interior were at odds. 辛い (つらい, 'painful, bitter, hard to bear') describes emotional rather than physical pain here. The contrastive が bridges the public performance and private suffering.