Student or Learner
In the back of my mind was Dorothy's voice, and when I spoke, I used her words.
At the back of my mind, I guess I was afraid that I might have given it to him.
Does "in the back" and "at the back" mean the same, please? If not, what's the difference, please?
in the back of your mind is an idiom which means: understood or known, it is in the memory and at times reflected in memory. So in the first sentence when you spoke, the voice of Dorothy is reflected.
At the back of my mind: it means: on the surface of the mind, not gone into the memory. So in the second sentence you are not sure of your action as it does not reflect in the memory.