I have a hard time throwing away things like that, too.
I read an idea once to take photos of things you want to throw away because the photos take up less space. I do it sometimes.
I still have my stuffed animal collection from when I was a kid, but when I have to move, I will take photos first, then discard the toys.
I think it's a great idea to have her best friend or your sister sit with you. They might even be able to offer insight.
One 'rule' I used when I was discarding my late husband's things... if I had an emotional reaction to the item, I kept it, if I didn't, I let it go.
I haven't cleared out his sock drawer yet... it's almost been 9 years.