。
I'm always looking only at you, you know.
Literal
I [topic-は] always you only seeing-am [explanatory-んだよ].
君ばかり ('only you') uses ばかり in its 'just/only' sense, similar to だけ but with a stronger nuance of exclusivity or even annoyance from outside observers. Combined with ~ている (ongoing) and んだよ (emphatic explanation with new-info marker), this is a romantic confession that frames itself as 'the situation is...'