。
You really are terribly forgetful, aren't you.
Literal
Truly you [topic-は] thing-forgetting [subject-が] severe [explanation-の]-[agreement-ね].
Classic double-subject construction: あなたは sets 'you' as the topic, then 物忘れが introduces a specific property within that topic — 'as for you, forgetfulness is severe.' This 'large-topic / small-subject' pattern is characteristic of Japanese grammar, where a single sentence can stack multiple argument-marking particles. 物忘れ ('thing-forgetting' = forgetfulness) is a native-word compound. のね combines the explanatory の with the agreement-seeking ね — feminine-leaning in tone.