Just a note on what is going on here -
The OP has identified a problem, namely a house full of accumulated clothing and other items, and all the stuff is having a negative impact on her quality of life.
I want to be free of my stuff. I’m drowning in it. My house is a mess and all that happens when I have a sort out is it gets moved about. I spend money on storage solutions when I need to just get rid.
People with this sort of problem have a mental health issue commonly known as hoarding. Hoarding is not healthy.
The solution to the problem is to take steps to stop the attachment to inanimate objects, the sense of obligation toward them, the sense that they are a part of your life and you owe them care and consideration.
Hence the advice to make the decision that they are no longer part of your life, you have no emotional attachment to them, get a skip, dump the lot, and move on.
Resolve to do better in future, don't buy stuff you won't use.
Anything else - all the faffing with this or that charity - is enabling the problem that lies at the base of hoarding, which is inappropriate emotional involvement with inanimate objects. Catastrophising about the impact on the environment of one more skip of clothing and knick knacks is inappropriate emotional attachment and is holding the OP back from making the decision and devoting a week to clearance.
'Perfectly usable' is hoarder talk.
What it means is passing on the problem of stuff you don't want so that it can fill up someone else's house and cause them misery. It is failure to move on. People using that language are keeping the OP trapped in the hoarder mindset.
The idea that she would be doing people favours by freeing up 'perfectly usable' junk for them to buy is a fallacy. It's just a way of avoiding decision-making, avoiding real change, and the hand wringing about the environment is another example of failure to make a break with her habits of mind around inanimate objects.
She must make changes to her mindset in order to shake to shake the mental load off her back. The best way to change is to decide it has no part in her life, no claim on her care, and to get rid of it all.