I think that you have to look at what the 'stuff' is costing you by being there:
Inability to use the space for something else
The time it takes to organise it, store it, clean etc.
The negative mental energy of feeling cluttered and untidy
Not fitting your changing needs.
Reasonable to keep:
Something for a sport or hobby that you use infrequently, but where it is an advantage for you to have your own eg. ski boots, sewing machine.
Good quality or unusual items of clothing that you have a reasonable expectation you will fit back into eg. I hover somewhere between a 10 and 12 for trousers/skirts etc, so if I have a good quality item in a 10 I keep it.
Genuinely sentimental items. But, review these annually so that you a) get the pleasure of looking at them b) cull things that no longer seem that important.
Not reasonable to keep:
Projects that you are definitely not going to finish - half finished knitting? Give the pattern, needles and wool to someone who will.
Clothes that neither fit you nor are particularly good quality.
'Collections' that you once made. I sold a collection of china animals on ebay. There were people out there who loved them and I just kept my favourite, a little girafe who is sititng on the shelf above me now. By keeping one item I have kept the 'memory' of the collection, but other people out there are appreciating them far more than I did when they were wrapped up in a box.
Things that can be stored in a more compressed form. We had a load of photographs professionally scanned in for us ( a company did it all - google if interested) and we look at them far more now than we did when they were locked away in albums.
Things that have never been to your taste but you recieved them as a gift etc years ago. Ebay, sell or charity shop it - who will really know?!
Duplicates. Two pairs of ski boots? Take a deep breath and pick your favourite.