区别就是两者意思是不一样具体的不同如下
on my mind中文意思是
在我心中;你永远在我心里;在我脑海;
例句
I've been trying real hard not to think about you all the time, but you're always on my mind.
我总想不再牵挂你,但你却无时无刻不在我心中萦绕。
in my mind中文意思是
在我脑海中;
例句
It's the end of the world in my mind.
在我脑海中,世界末日已经到来。