I really don't mean to be cliche or repeat from my fellow posters but my biggest stumbling blocks are *ta da* Time and Doubt.
Time, where does it go? I don't have kids yet, but I am blessed with a demanding job and hubby. I start my day with my drive to work, with my ipod playing all of my favorite songs, thinking about scenes I want to work on that day. I promise myself that I WILL write when I get home. I really will. But then I get home and finish up all my stuff and I'm pooped. Those brillant ideas that floated through my head just 8 maybe 10 hours ago, up and floated away. When I'm having a particuarly good day, I write those ideas down when I get to work. But again, sometimes, I hit the door running and that good intention gets lost.
Doubt, sheesh, that's a big one. I often feel like I'm in no league with my crit buddies. They are all brilliant and I'm drivel *smile* That freezes me. Every idea or scene I'm working on, I drop. Fortunately, my characters and my love of the written word drives me out of my pity party. Who cares if I stink? As long as I have fun and can be happy with the work I produce.
So my horse wears a life preserver! I am lucky. I have great friends who push me to be better and encourage me during the low times. They act as my floatie. Coupled with my own need to accomplish my dreams and finish a project, I manage to keep swimming.