Hi, Tony C,
What is the difference between on my mind and in my mind? Can they be used interchangeably? Thanks!
'In my mind' could mean 'in my opinion' or something that exists in my imagination like the common expression 'in one's mind's eye'. Also, you can use it when you are thinking about something or somebody without feeling worried about it/them. Another expression is 'keep in mind' which means 'remember'.
'On my mind' means to be constantly in one's thoughts; of concern to one. It refers to something worrying or distracting you. A similar expression is 'to weigh on one's mind'.
- The approaching trial was on his mind. (Source: https://www.wordreference.com/definition/mind)
See this thread and try to understand David's reply: