、！！！
You've got this — believe in yourself! You're already a wonderful swimmer!
Literal
It's-fine, you [if/as-for-なら] can-do! Self [object-を] believe-[request-て]! You [topic-は] already already splendid swimmer [explanatory-の][feminine-よ]!
A coach-style pep talk. Writing キミ in katakana (instead of 君) lends a slightly warmer, more stylized register typical of fiction or media. もう既に stacks two near-synonyms for 'already' — a common emphatic doublet. なのよ at the end is の (the explanatory/adnominal nominalizer) + feminine よ, often used to convey 'you see / let me tell you' emphasis. 信じて is the bare te-form functioning as a softened imperative.