「私にはあなたしかいないの。あなたがいれば他（ほか）には何（なに）もいらないわ。」...maybe I read too much girls' comics XD Note that the translation can be totally different depending on the speaker and the listener's gender. In the case of mine, "I" is female and "you" is male. In Kirius-san's one, "I" is male and "you" is female. alucky-san's translation can be used by both, I think.

well depending on how cheesy you want it to sound, maybe this will work for you: 僕の心は君でいっぱいだ、もう君以外誰も受け入れない

私はあなた一筋（orあなたに一途 ）で、あなたは私の全てです。 something like that.