If you search this forum you'll find a lot of information about this from other questions. Basically, 'in mind' - something I am thinking about 'on my mind' - something I am worrying or fretting about. Something I'd rather not have to think about.
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Davidrock65
Whats the difference between in one's mind and on one's mind?
A. That's what she had in/on her mind. (My guess is that it woul be "in")
B. I will always keep you in/on my mind. (My guess is that it would be "in")
C. What's on/in your mind? -- The answer is on