Why is it so hard to let go?

I need to do a better job at donating clothes that I can't fit, don't wear, or bought on impulse [don't really need].

It's hard to let go of what I can't fit because I'm convinced I'll lose weight and be able to wear it again. There are some items that I have a sentimental attachment to. And the impulse purchases are past the return window.

Maybe I should consider hypnotism so I can be able to Just Let Go.

Am I alone in this? How do you break the attachment to things that aren't really serving you?