at/in the back of your mind
in the back of (one's) mind
In one's thoughts, but with little or no conscious or active consideration or reflection. Despite my success at work, there's always this worry in the back of my mind that I'm doing something wrong. You don't need to do anything about it now—just keep it in the back of your mind going forward.
at the back of your mind
COMMON If something is at the back of your mind, you are aware of it, though it is not the main thing you are thinking about. It was not until they had eaten that Brigadier Ryle raised the subject that had been at the back of his mind throughout the meal. Always, at the back of my mind, was the fear that the depression might overcome me again.