This stuff can also come about when a) You're a perfectionist, so you run thru in your mind what it's about, but when it comes to doing 'the big job' well you've already done it haven't you, it can't compete with your ideal draft version.
Way round this? Remember that often as you write new stuff comes to you, so when you get it down chances are all sorts of new goodies will occur to you in transit, so your draft version in your mind is just that.
b) Life has got on top of you. Putting stuff off is a way of exerting control, just as a messy room is your way of telling the world you can do as you please, just as anorexia feels to the victim like taking control of your body in a world where generally you just don't feel in control at all, just as morbid obesity comes from someone having their cake and eating it, really 'owning' their food, it's a feeling of control. Of course, you wind up with a messy room and not the means or wherewithal to sort it, stick thin and dying, or carrying around a few extra stone of weight each day year in year out; what started as a desire to assert control only ends up as an outward manifestation of your lack of it.
c) You have put off the deed it is now in the part of your mind called 'on the backburner' as separate as fact and fiction, past and present. Also, let's face it, if you didn't do it last month, you surely don't need to do it this month and if you didn't get round to it last year, surely the chances are slim you will get around to it this year.
"This is where we leave you Mr Bond."
Roger Moore 1927-2017