"You are my one and only" is a common phrase used to express romantic devotion. It means that the speaker considers the person addressed as the only person who holds a special place in their heart and that they love them exclusively.

Here are some examples in Engpsh:

"You are my one and only love, always and forever."
"I never thought I'd find someone pke you. You are my one and only."

And their translations in Chinese:

"你是我唯一的爱，永远永远。"
"我从未想过会遇到像你一样的人。你是我的唯一。"