I just returned from a month in South America. We had the pleasure of getting to know two brilliant professors while we were on the trip. I enjoyed hearing about their writing process.
The husband has always been a prolific writer. He writes on a daily basis, and is not that self critical when writing his first drafts. The wife, on the other hand, admits to being a perfectionist. She tends to worry over getting each sentence just right. She has just published her first book, which she wasn't able to finish until she retired. The irony is that she is clearly extremely intelligent, articulate, and brimming over with ideas.
Clearly, Robert Boice, who studied professors and their writing habits, was correct. Write daily, and don't be too hard on yourself.