。
She thought it would be hard to adapt to the new environment.
Literal
She [topic-は] new environment [to-に] adapting [nominalizer-の] [topic-は] difficult [quotative-と] thought.
A complex sentence with multiple layers. 順応する (じゅんのうする, 'to adapt, to acclimate') takes に for what one adapts to. The の nominalizes the whole clause, は marks it as the topic, and 難しいと思った reports her internal judgment via the quotative と. Note the double は: the outer は marks 彼女 as the sentence topic, while the inner は marks 'adapting to the new environment' as the topic of the embedded clause.