I've seen 'what I had in mind' , 'what was in my mind', and 'what was on my mind' quite often, but I haven't seen 'what I had on my mind' as often.

Why do you native speakers think 'what I had on my mind' is rare?
I understand "what I had in mind" as being more projective, more like "what I meant", "what I was aiming at"
"in my mind" is something I can't/don't want to forget,
"on my mind" is heavier, not an obsession, but something recurring.

but, Taka, wait till a native reads your post!
MrP, could I have your comments, please?
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He should be along soon, Taka. He has other things on his mind at the moment, perhaps.
He'll probably bear it in mind though, Mr. M Emotion: smile
Hello Taka

The 'in my mind' idioms relate to 'what I was thinking of'; whereas the 'on my mind' idioms relate to 'what I was worrying about', or 'what I was preoccupied by'.

Perhaps we often have to explain 'what we had in mind', or 'what was in my mind', because those can relate (for instance) to plans or ideas that affect other people:

1. What I had in mind was blue for the window frames and white for the doors.
2. I can't explain what was in my mind at that moment; I just hit him.

Similarly, we may sometimes have to explain 'what was on my mind (at the time)', because 'what was worrying me then' may affect other people now:

3. It was very much on my mind that the project wouldn't be completed before the autumn.
4. It's true that, one by one, I seduced the entire English Ladies' Football team; but you were always on my mind.

'What I had on my mind', on the other hand, may be less common because it seems to relate not so much to a continuing state of preoccupation (e.g. in #3, worry about the project), but to something complete. I'm not sure there would be many occasions where we wanted to talk about a completed period of preoccupation. Even here:

5. I wasn't going to tell MrP what I had on my mind.

This would seem just as natural:

6. I wasn't going to tell MrP what was on my mind.

But I'd be interested to hear other members' thoughts on this.

(I notice that many of the googles for 'what I had on my mind' are mistakes for 'what I had in mind', by the way.)

Students: Are you brave enough to let our tutors analyse your pronunciation?
Thank you, MrP!