in my mind v.s on my mind

[Juzwannabebetter]

Which one is correct?! And Why?!!?!?!?!?

[Untaught]

If somebody is on your mind, you are thinking and worrying about them/it a lot.
Example:
You have been on my mind all day.

[Juzwannabebetter]

How about in my mind?!

[Cerberus™]

I think the difference between in your mind and on your mind is a bit random, I doubt there exists an all-explaining rule for it.

Something is on my mind: you are simply thinking about something. More general.

Something is in my mind: more specific, you may be actually picturing something in your mind. You treat the mind as some sort of abstract container of ideas or images. But the difference is very subtle; in many contexts either preposition can be used.

Do you remember this song of Schubert's we listened to last week? - Yes, I can still hear it in my mind.

... and in this landscape there were giant rocks the shapes of angels. - Hello Alexander, nice to see you; what were you talking about? Where was this landscape? - It was just in my mind: I was telling her about this dream I had.

In addition, there are countless idiomatic expressions with "mind"; you will find them in a dictionary.