For me the biggest problem with journaling is that I spend *too much* time on it. I journal before going to bed, and I often start doing so with the intention of spending 15-30 minutes on it. 3 hours later, it's past midnight, I've written 2000 words, describing and analyzing every minute detail of the day, and I still can't go to bed because I have so much more left to write.
But, as I point out every mistake and every wrong decision I made that day, I do improve at a very rapid pace when I journal. Maybe, after a few weeks of journaling, I'll make so few mistakes that I'll be able to keep journaling this way but be done in 15-30 minutes.