Oops...

July 5th
It's a rainy day again today.

Oh no! I've left my textbook at the office! I have a lesson tomorrow, but I can't enough prepare. (I am not prepared? I'm not prepared enough? Either one could fit!)

Well, Maybe I should go to the office to get it. (you could also say, "Well, maybe I should go back to get it.") Of course I know...but I won't go to my work place on a holiday! Mmm... What should I do!?

Good journal entry! run to get that textbook before your lesson ∑(;°Д°)

Oops...

July 5th
It's a rainy day again today.

Oh no! I've left my textbook at the office! I have a lesson tomorrow,but I can't prepare enough prepare.
Well, maybe I should go to the office to get it. Of course I know...but I won't go to my work place on holiday! Mmm... What should I do!?

[Good work.  Usually we use 'that' to specify which one of something.  In this case, we know which one, so 'it' is more appropriate.]

Oops...

July 5th
It's a rainy day again today. (Not again!!! really?  seems every summer day is a rainy day!)

Oh no! I've left my textbook at the office! I have a lesson tomorrow, but without it I can't enough prepare well enough/do enough preparation.
Well, maybe I should go to the office to get that. Of course I know...but I won't go to my work place on the weekend/a holiday! Mmm... What should I do!? (if you need it, just go - it really doesn't matter that it is the weekend).

Here we don't think of the weekend as a holiday, just non-working days

Oops...

July 5th
It's a rainy day raining again today.

Oh no! I've left my textbook at the office! I have a lesson tomorrow,but I can't enough prepare enough.
Well, Mmaybe I should go to the office to get that it. Of course I know...but I won't go to my work place on holiday! Mmm... What should I do!?

- Minimal revisions. Way to go Mizuki-san!