From today on, I can’twon't be able to write a long English diary likeentries like I did during the Christmaswinter holidays. From today on, I won't be able to write long English diary entries like I did during the winter holidays.

"from today" isn't really something we say, instead you can use "from today on" or "starting today" or "from today onwards". In a business or at work, they often say "Effective today", instead. Also, I changed the tense from present to future, since you are talking about something that is going to happen in the future (in the future, you won't be able to write long diary entries). "a long English diary" sounds like you mean a whole entire diary, so hundreds of pages long. I think you meant just a single diary entry or page in your diary, so that's why I changed it. I can't remember hearing people say "Christmas holidays", it didn't sound natural to me. I think "冬休み" can be translated as "winter holidays" or even "winter break" if you are talking about a break/vacation/holiday from school.

FromStarting today, I can’t write a long English diary likepassages in my diary as I could during the Christmas holidays. or, more commonly, Starting today, I can’t write as much in my diary as I could during the Christmas holidays. Starting today, I can’t write long English passages in my diary as I could during the Christmas holidays. or, more commonly, Starting today, I can’t write as much in my diary as I could during the Christmas holidays.

We usually just compare something between times as "more" or "less."