"On my mind" and "in my mind" are both expressions related to thoughts and emotions, but they are used in slightly different contexts and convey different nuances: On my mind: This phrase is used when you want to express that something is currently occupying your thoughts or preoccupying your mental state. It often refers to something that is concerning or bothering you. When you say, "X is on my mind," you mean that the subject or issue is actively present in your thoughts and is affecting your mental focus. Example: "Lately, the upcoming exam has been on my mind, and it's making it hard for me to concentrate on other things." In my mind: This phrase is used to describe something that exists within your thoughts or imagination, but it might not necessarily be causing any immediate concern or preoccupation. It can refer to ideas, thoughts, memories, or images that you are contemplating or visualizing in your mental space. Example: "In my mind, I often picture myself traveling the world and experiencing different cultures." In summary, "on my mind" suggests that something is actively occupying your thoughts and possibly causing concern, while "in my mind" refers to thoughts and ideas that exist within your mental realm without necessarily indicating a strong emotional impact.

Look at the prepositions "on" and "in". They are used to describe location. If the cat is lying "on the table" it means it is on top of it. The table is supporting the cat. It is resting or standing on it. If the cat is "in the box" it means the cat is inside of the box. The box is holding the cat and perhaps the cat can't escape from the box. What does this have to do with thoughts? Well, it's the same really. If a thought is "on" your mind, then your mind is supporting the thought. It just means you are thinking about the thought. If a thought is "in" your mind then your mind is holding it, and perhaps it cannot escape. Here is an example: "In my mind I'm going to Carolina": this is actually the name of a song. (Listen to it!) It means that captured within my mind is the thought of going to Carolina. The thought can only be there in my mind. If I let that thought loose and told other people what I was thinking they might laugh at me because, obviously, I am NOT really going to Carolina. That thought exists only "in my mind". "In my mind" is similar to "in my imagination", but not the same because perfectly logical true things can also be in your mind.

Ahmadi gives a good explanation. I would add a little to his explanation. On my mind means that you are thinking about something. There is a song "You were always on my mind." The singer is saying that he was was always thinking about someone in a romantic way. On my mind doesn't necessary have a negative or positive implication. The context will generally tell you if it is a good thing or a bad thing. In my mind generally indicates something that exists in your head but doesn't necessarily mean that it exists in the real world. I think he is plotting against me but, that could just be in my mind (in may head). You think that the person is plotting against you but it is possible that you are just jumping to a conclusion that may not be real.