expression
preoccupied with; obsessed with; only thinking of
Idiomatic expression meaning one's mind is completely filled with a particular thought or worry. Often used with ～で or ～のこと.
明日の試験のことで頭がいっぱいだ。
My mind is completely occupied with tomorrow's exam.
彼女は新しいプロジェクトのアイデアで頭がいっぱいだった。
She was obsessed with ideas for the new project.
Literal meaning 'head is full'; idiomatic extension to mental preoccupation.