。
She had been gradually fading from my memory.
Literal
She [topic-は] my memory [from-から] gradually fade-was-on-the-verge-of-doing [progressive-ていた].
薄れる ('to fade, become dim') is built around the i-adjective 薄い ('thin, light, faint'); the verb form means 'become thin/faint.' The compound 薄れかける uses the auxiliary ~かける ('be on the verge of, start to'), giving 'on the verge of fading.' Combined with the past progressive ~ていた, the result is a delicate temporal layering: she was, over time, in the process of nearly fading away — a subtler tense than English easily captures. 次第に reinforces the gradual nature: 'little by little.' This is sentence-as-poetry: simple components yielding a wistful, almost cinematic image.