On my mind vs In my mind

Can anyone give me an example that illustrates the difference?

For instance, Pet Shop Boys says "You're always on my mind". But in some other occasions people say in my mind.

 

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Mostly, we see or imagine things in our minds, and on our minds are things we worry about or concern ourselves with.

I can still see in my mind how you stood gazing forlornly at the empty closets when I left you, and the unhappiness I caused you is always on my mind.

 

http://www.englishforums.com/English/OnMyMindVsInMyMind/bpvxj/Post.htm

 

 

출처: http://blog.daum.net/bizncs/15932789