Advice for decluttering within collections?

I collect books and kpop albums (and by extension, kpop photocards & album inclusions) but outside of those collections, I'm pretty minimalist in both aesthetic and philosophy, and I declutter often. I live in a fairly small space and am maxxed out on the number of bookshelves I can fit, and they have gotten full to the point that they are starting to feel overwhelming and look overcrowded, which is usually my sign that I need to go through and downsize and declutter. So I started trying to do this and immediately just... hit a wall.

I've done this many times through the years, but in the past, I've had a fairly easy time going through my collections and deciding what to keep and what to sell, but this time I'm finding myself just staring at everything, unable to make a decision. It's almost like I'm unable to tell what matters to me anymore. Do I still want to keep that book I gave 4 stars 5 years ago and haven't thought about since? idk, maybe? What about that photocard of one of my least favorite members of my favorite group? No idea. Do I actually still want to read those unread books that are still sitting around? I. don't. know.

It's like I've been casually collecting a certain way for years and then suddenly took a step back and can't remember why I'm even doing this. I almost feel like I could happily get rid of all of it today, but then would almost certainly regret it immediately.

Have any of you ever had this problem? And if so, do you have any suggestions for figuring out what actually still matters to you within your collection?