。
She did that not for your sake but for her own.
Literal
She [topic-は] your sake [not-ではなく], own sake [for-に] did-so.
The contrastive frame ~ではなく~ flatly negates one possibility ('not for your sake') and asserts another ('for her own'). 自分 reflects back to the sentence subject, so 自分のために unambiguously means 'for her own sake' — the speaker isn't talking about themselves. そうした is the past form of そうする ('to do so/that'), where そう is a deictic adverb pointing back to a previously mentioned action — Japanese leans heavily on this kind of pronominal-style reference rather than restating verbs.