An idiom meaning to be preoccupied or obsessed with something; to have one's mind full of only one thing and be unable to think of others.
試験のことで頭がいっぱいで、食事のことまで考えられない。
I'm so preoccupied with the exam that I can't even think about meals.
恋愛のことで頭がいっぱいな友達に、冷静になるように言った。
I told my friend, who was obsessed with love matters, to calm down.
頭がいっぱいでメモを取るのを忘れてしまった。
My mind was full and I forgot to take notes.
問題が多くて頭がいっぱいだが、一つずつ片付けていこう。
I have so many problems on my mind, but let's tackle them one by one.