The correct sentence with a gerund as the subject is; Remembering all of the details of the accident was difficult for her.
Gerund: It's use as Subject of sentences
A gerund is a noun made from a verb word (usually termed the root) plus ing (a present participle). On this note, a whole gerund phrase functions in a sentence just like a noun and can act as a subject, an object, or a predicate nominative.
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