Looking at it is daunting.
I'm both proud and relieved to say that my things amount to, at most, a quarter of what you see in the photo, but taking the 100 Thing Challenge is forcing me to be honest and keep an accurate count of my possessions. So I have begun reviewing what I added to the pile.
I started with an 18 gallon bin of clothing that I haven't worn recently, either because it doesn't fit or because it's better suited for warm weather. While sorting through it, I realized that I had made attempts to get rid of some pieces before, but inevitably and repeatedly found reasons to keep them. This time I was more strict, and filled two trash bags with donations. What I kept is accounted for on my most recent list, and fits in less than half of a carry-on suitcase.
It sounds simple, but the sorting actually took a lot of time. I made three piles:
Keep it
Donate it  
OK maybe I'll keep this too
Eventually I saved one item from the maybe pile, and dutifully did away with the rest. As I went through the clothes again and again, I was reminded of how much money I had spent on stuff I was not using and did not need, and that what I had actually left in my closet was more than enough.
Later that day, I visited a mall to pick up two items I needed. Though I took some time to stroll around, and even visited a few stores, I didn't feel the need to buy anything. I had a much more acute sense of how careless I had been before, and that all my needs are being met. There is nothing out there I can bring home from a store that will make my day any better. So I found and bought what I came for, and then met good friends for dinner.
Letting go of possessions is hard. You spent money on your things. Some of them are perfectly fit for use. But you have more than enough. See that you don't need to spend so much. Find a good home for the things that meant something to you, and donate the rest to others who may need it.
The choice to go minimal is scary at first, and getting there can be difficult. But the harder you work at it, the more satisfied you will be with your progress, and the more appreciative of the change you've made.