You feel like you're giving away Grandma instead of something that once belonged to her. I've done that, many times.

Karen's advice is spot on -
When you see an item you no longer use, instead of saying "that's from Grandma's house," and putting it back, think "That's from Grandma's house. Think how happy she will be to know it is going to someone who needs it, and will put it to good use!"
And kitten's suggestion about putting things in a sealed box for a period of time has worked for me. If you don't need it, use it, even wonder where it is for 6 months - get rid of it.