What helped for me was using eBay and Amazon as I went through my books. If I could buy it back at a reasonable price, then it was easier to let it go now. I had all my quilt books inventoried and moved everything I donated to a subset, so that when my brain says “You have that book”, I can verify whether it’s still on the shelf or not.
It also helped that I had a goal: get rid of 1 of the 2 bookcases to make room for my new sit down quilting machine. Still, it was an emotional process and I just had to recognize that.