"On my mind" or "in my mind"?

I certainly have this issue on my mind or in my mind to reply to you. Which one is correct? What is the difference?


‘On my mind’ suggests that you are thinking about something and usually this matter is causing you some concern/worry as in:

He has a lot on his mind at the moment what with his recent house move and his money problems so he can’t see you at the moment.

‘In mind’ is the usual expression suggesting something you are planning as in: I’ll tell you my plans for the new house and what else I have in mind to do in the meantime.