get something into your head idiom
开始相信
to start to believe something
When will you get it into your head that he's not coming back? 你什么时候才能相信他不会再回来了呢？
One day, she got it into her head (= decided for no reason) that we all hated her. 有一天，她无缘无故地认为我们都恨她。