。
She is gradually fading from my memory.
Literal
She [topic-は] my memory [from-から] gradually is-fading.
薄れる ('to fade, become dim') is built around the i-adjective 薄い ('thin, light, faint'); the verb means 'become thin/faint.' Combined with ~ている for an ongoing process, the sentence describes someone slowly receding from memory rather than vanishing in one moment. The kana spelling しだいに (rather than 次第に) is a stylistic choice that makes the sentence feel slightly softer, less academic; the meaning is identical. The result is a graceful, melancholy line — each day she's a little dimmer in the speaker's recollection.