Why do I find it so hard to get rid of stuff?

Over the last few days I have been trying to clear out old 'stuff' in order to make way for the new 'stuff'. However, I have realised that it is really difficult. I seem to find a multitude of excuses to hold onto old clothes for example. Some of these are ones I will never wear again but I feel I need to keep in case I need to make something out of the material, or I can use as cloths for cleaning. The reality is that I have enough 'old' cloths for cleaning, and a stash of new material for making things out of. So, why then, can I not throw these things away? or donate them? I have managed to sort my clothes into three piles: rags, donate and keep (but hide away somewhere until I decide what to do with them). Surprisingly my 'keep' pile is the biggest. Aghgghghg. I know I want to get rid of these things but something holds me back. HOW can I do it? I do occasionally have a one in - one out policy, but that has somewhat taken a back seat.... perhaps it needs to be reinstated once I have gotten rid of these 3 piles. Oh! Why do I feel the need to keep 10 plain black t-shirts?