what does I have not "used my diary" mean? I have not ever seen this before.

yeah, breakfast is not the cause. So I say "before you go to work, eating food is important". If you don't eat anything before you start to work, your engery will probably not enough to support your all-day work

When you wrote "kept" in this sentence it means that you did not keep it indicating that you for example; lost it, threw it away, etc. The word "used" is used because this instead indicates that you still had your diary in your possession, but did not write in it.

oh man, I expexted that you might say that before you explained to me.
Does "Keep my diary" mean like "I have a book"?
What I want to say is I "have not written entries" for a period of time.

Are Keep my diary and Keep diary the same thing? Textbooks taught English learners that keep diary means write entries. However keep diary seems to be not as the same as write diary for English native speakers.

"Keep my diary" as its own sentence would be a statement you would say to someone. For example, you tell someone to keep your diary, that means you want it to stay safe with them. If you're instead referring to your sentence "I have not kept my diary" for example, it means that you lost it. When you say you have a book, it means that you have it in your hands or somewhere else. This does not mean you have it with you right now, you're just saying you have it in the kitchen or wherever it maybe.

If you want to say "have not written entries" then you would write "I have not used my diary for some time". You could also say "I have not written my diary for some time".

I did not know what you were talking about "Keep my diary and Keep diary". Since, I've never heard anything like that until I Googled it. After Googling, I figured out that "I kept my diary" grammatically works. But, the truth is, majority (if not all) of native English speakers would immediately think of you keeping your diary in your house, not actively writing in it.