BBC Learning English message boards

My question:

What's the difference between in/on one's mind?
And how to use them?

Teacher's answer:

They are both correct, but are used in different ways. For example:
1) "You are always ON my mind." "What is ON your mind right now." "I have the 'Theory of Evolution' ON my mind".
BUT,
"Even though we are far apart, IN my mind I can visualize you, I can touch you, I can feel you, and the only thing I hear is the beating of your heart."

Reflection:

Sometimes the English preposition usage like in and on is confusing. The sentences like in my mind and on my mind often can be heard, so I wonder there are differences and how to explain the differences.

I am glad to see that teacher provided several examples, but I think it would be better that the teacher could provide more clear explanations or rules.

We also have the board for asking English question in Taiwan, but the answers are offered by anyone on the internet, while the answers in the BBC Learning English message board are offered by professional teachers, which I think is more convincing and accurate.

However, because of time differences, the board will be closed in a certain period and my question could not be asked whenever I want.