I’ve written in this diary for nearly onea year.

Actually, every theme I’ve written comes from the book named “Q&A diary”, so I could simply enjoy writing every day.

Sadly, the frequency of my post might decrease from today, but I’m going to keep writing.

I’m gladll be happy if you willcould help me sometimes.

My (one year diary.long diary) / one year anniversary

"one year long diary" focuses more on the diary itself being ongoing for a year, while anniversary focuses more on the specific day that's 1 year after starting.

Actually, every theme I’ve written about comes from the book named “Q&A diary”, so I could simply enjoy writing every day.

However, I just finished the book today, so I have no idea how I will continue.

Sadly, the frequency of my post might decrease from today, but I’m going to keep writing.

Actually, every theme I’ve written coame from thea book named “Q&A dDiary”,” so I could simply enjoy writing every day.

Since you haven't mentioned this book before, use "a" to introduce it
CAVEAT: you can say "Every theme I've written came from the book "Q&A Diary." The nuance here is that you're assuming the book is famous enough that someone will already know what it is, without you having to tell them what it's called (hence you don't say "the book NAMED "Q&A Diary")

However, I just finished the book today, so I have no idea how I will continue.

Sadly the frequency of my posts might decrease fromafter today, but I’m going to keep writing.

I know in Japanese it's 今日から but we tend to say "after today" instead. "From today" is uncommon. You could say "from today onwards," "from now on"

I’m'll be glad if you willcan help me sometimes.