Don't Punish Yourself
Start a daily journal habit. But if you miss a few days, just write about today, don't punish yourself by trying to fill in the blanks.
This goes for just about any daily habit. Keeping track of “streaks” is a good thing, and often motivational. But if we lose sight of what we're trying to accomplish and focus on the “streak” we can start to de-motivate ourselves.
Now that I'm out of school I am trying to lose the “I need to get straight As” mindset. A “perfect attendance record” isn't as important as the improvements I make when I work on things. “Write every day” is a good way to be a writer, but there's not a score card for writing every day. The important part is the writing, not the days. If I miss a day, all that is really lost is the opportunity I had to improve that day, nothing is taken away from my ability to improve today. Punishing myself for “screwing up” or “missing days” is counter-productive and needlessly costly.