Oh hon, you aren't alone. When I packed up my mom's house there was a TON of things I kept of hers and dads, not to mention boxes of my own stuff that I left there. At the time, a lot of it was sentimental. Now I'm finding myself unpacking something and saying to myself "why did I keep that??". A lot of it is perspective.
It's fine to still keep some things in boxes forever, like year books, photos...things that you can't get rid of but don't need to see everyday. That way if you do want to get them out you can, but they aren't taking up space. For things that are sentimental that you do want to display and are out of room, think about individual wall shelves. The one shelf that mounts on the wall (where the cats hopefully won't be able to get to!). That way it can be displayed, you will be able to see it, but it won't be taking space away from everday things that you use.
Good luck! I still have a whole shed of boxes to go through, and I'm not even sure what is in most of them!