The Rolling Stones sang, "Time is on my side." I don't often feel that way when I'm writing. It just seems like there is never enough time. I used to think that I had to have hours of uninterrupted time to get some writing done. False!
Sure, it's easier if you have a nice, long stretch to get that momentum going, but I've found that I can get something done in ten or fifteen minutes, even if it's just editing a couple of pages, jotting down ideas, or adding just one new paragraph. Thinking that I can't is a pitfall that I've encountered many times and still deal with regularly.
If I just sit in the chair and put my fingers on the keyboard I'm half way to getting something done. One thing I've found that helps is setting these smaller, more realistic goals rather than the grandiose vision I usually have in my head when I wake up in the morning about how I'm going to finish a whole pass today. That, and actually seizing those little segments of time we get throughout the day and making the most of them. It doesn't have to be great or long, or well developed, it just has to be something. And little somethings add up.