what helps me is to think that the clothes are being worn by someone else, rather than think of them being "thrown away" or lost. Yes, you might like it and value it, but there is someone else there, missing out because you haven't parted with it.
Or you could think of your wardrobe as an archive..and take pictures of everything, put them in an album and then select one in three items blindly to keep. The thing is, if you keep buying new clothes and wearing them too,the old ones might be in very good nick too, which makes it even harder to give them away as they are not old and shabby...and the greater the feeling you are wasting something valuable.
But you are not, you are creating space in your life for new purchases, or maybe reminding yourself that you have too many clothes, and that the money could go on something else you like, to treat yourself.
I am not a clothes hoarder by any means, but I do find myself remembering things I have given away (there was a beautiful embroidered silk vicose emerald green skirt from M & S I used to wear with a cappucino coloured linen jacket - both gone because they didn't fit me anymore) and then I just think, yes, I can remember it without having it. It is still fresh in my memory. And my absolutely favourite slinky red and white dress from Laura Ashley I wore to every party in my 20's. It doesn't look good now, so in fact memory is better than having it sadly in my cupboard.
The other thing that helps is to stop buying new clothes, and just wear the old ones
After a bit you get so bored of the old ones you long to throw them away. That has worked for me in the past! -or getting fatter and hair getting greyer and certain styles no longer suiting