。
I can't stop thinking about you.
Literal
I [topic-は] you [possessive-の] thing [only-ばかり] am-thinking.
君のこと ('about you' — literally 'the matter of you'). ばかり ('nothing but, only') emphasizes the exclusive focus on the beloved. ~ている marks the ongoing state of constant thought. A classic expression of romantic preoccupation.