"On your mind" means that there is something that you have been thinking about. Quite often it means that something has been troubling you, but it can also mean an idea, or perhaps someone you are romantically interested in - they are "on your mind". It's common to hear "Have you got something on your mind?" (meaning, is something troubling you, or is there something you want to say?)... for example. "In your mind" - this is less common. We use it to put emphasis on the "your" or the "my" to indicate a different way of thinking from someone. For example, "In MY mind..... it's like this". It's more common to say "In my mind....[I think this...]". It would be a bit patronising to say to someone "In YOUR mind....[you think this...]" unless you know them very well! I hope this helps : )

You are on my mind = I am thinking about you, probably worrying. You are in my mind = you are imaginary, you only exist in my head.

The first is incorrect