My English is pretty lame, Megumi, but it sounds correct to me, you know...

"I've made up my mind, but that's still a secret..."

You're just saying you've just decided on doing something or whatever, but nobody knows about it.

Well, sounds fine to me...but you'd better wait for a native to give you a more reliable answer. Hope that helps, anyway!

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Is this sentence correct?

" I've made up my mind! but That's still secret ! "


..Does this sentence make sense..?