I am joining this thread as an 'almost' reformed hoarder, but one with an interest in the psychology of clutter, hoarding etc.
My own hoarding history is around sentimental items (drawings, ornaments, school stuff, toys, letters) and mostly took place in my teen, student years and early twenties. I can see why it happened. My parents were war babies and my dad very much liked to stock up on packets of biscuits, tins of fruit etc. We would go to the supermarket at the weekend and buy 8 or 10 loaves of bread for the freezer. Sensible enough, but probably giving us children the message that it is good to have lots of stuff in case of emergency. On the other hand, my mother was very much into creative play and liked us to have lots of creative objects, craft materials etc. Our house was large and we had plenty of storage space for whatever we wanted. On the other hand, the family didn't have a large cashflow, so she would keep clothes, toys etc stored away for when we would grow into them. Not to mention enjoying a trip to the bric-a-brac stall - as did we all!
By the time I was in my mid-late teens I was aware that I had quite a lot of stuff compared to my friends, but it still wasn't a problem. However, when I was about 19 my parents began to talk about downsizing and, fairly suddenly, sold up and we moved into a smaller rented house while their retirement house was being found. Once the move was imminent, I literally had to pile all this sentimental childhood stuff into boxes. A few went to the retirement house and quite a lot came with me to my student room in halls of residence. I literally had a pile of boxes in my room with a few shawls thrown over them. 
These boxes came with me to my first flat, then to my second flat, where they stayed unopened. I never had time to go for the big sort-through and it became harder and harder as time went on, mostly because one of my parents was now terminally ill and I didn't want to stir up memories.
In the end I bit the bullet just before my wedding (age 27) and hired a de-clutterer to help me tackle those boxes. It was painful but worth every penny I paid her. I had just realised that I didn't want to move those boxes to yet another place I lived and somehow felt that I should start my married life free of them.
From that point on I began to make conscious decisions about what to buy and keep. My sentimental items are now in 2 neat IKEA boxes. I have charity-shopped so many clothes that I actually don't have enough trousers to wear. I still have a lot of old files to tackle and then need to start on some household stuff, but I feel it is under control. I can now begin to imagine a day on the horizon when I will feel that I have everything in its rightful place. I am now also very conscious of other people trying to offload stuff onto me - my sister is one for this since I had a baby - but if I don't want it then I just say no.
I think until you begin this process it is difficult to understand just how destructive too much clutter can be. I hope no-one minds me pointing this out, but a lot of people on this thread have mentioned being skint. Sadly, a lot of this stuff cluttering up our homes had to be paid for at one point or another... Is there a relationship between clutter and not having enough money for the things you really need?
I really support everyone on this thread and am behind what you are trying to do.