Final answer:
The sentence exemplifies a metaphor, comparing life's struggles to the act of swimming to convey perseverance.
Step-by-step explanation:
The sentence 'Things became hard in her life, but she just kept swimming' demonstrates a metaphor as a figure of speech. In this context, swimming is not meant to be taken literally but rather symbolizes persistence and continuing to move forward in spite of difficulties, much like how one would keep swimming to stay afloat and progress in water. This usage suggests a comparison between navigating through life’s challenges and swimming against a current, without explicitly stating it.