it's fear of change I reckon. Anxiety about moving on, wanting to burrow into my safety coccoon, all that stuff.
I can totally relate to this point.
I'm a terrible procrastinator and self-sabotager, and in recent years I've pretty much purposefully ground my life to a halt in many ways. I think accumulated clutter is a real comfort thing.
The comfort for me is manifold: (1) regarding the past: not sorting through a lot of this old stuff allows me to avoid bringing up painful memories. (2) regarding the present: the current clutter is a convenient excuse not to invite people 'round, which allows me to avoid letting people get too close, unless it's away from my flat and away from me. And (3), regarding the future: de-cluttering becomes this seemingly insurmountable task which needs to be done first, before I can really get on with the rest of my life; this huge convenient barrier.
The anxiety is caused by the guilt I feel about the current state of my flat, and also, as you say, the fear of finally having no excuse left to not shed this current skin, not to let go of the past, and not to move forwards into the unknown.
This sounds crazy to me now, but when I was seriously depressed a few years ago, I used to really worry that if I ever actually managed to get my flat properly de-cluttered and tidy, my subconscious motivation might've been working towards 'putting my affairs in order... everything's now in it's right place... now, time to die...'.
Also, one of the things that kept me going back then was the shame of somebody else having to sort through all my clutter! -All rather fucked up, I suppose. 
Anyway, lots of de-cluttering tips in this article: http://www.guardian.co.uk/theobserver/2000/jan/23/newyou.life1
It's always difficult to start tackling clutter, but tbh, on the rare occasions that I've had a go, I've usually found that once I've begun, I do actually get really into it with a vengeance. -Work methodically through the various areas of your house/flat, and be ruthless!
In the meantime, I know I've got a book called Dejunk Your Life somewhere in this flat, but honestly, I CAN'T FUCKING FIND IT!!...

Good luck BK, and good luck all.


I am a changing woman and I aim to have the place spotless in a few weeks. 