@Dragonhelm
I had this exact problem, but think I'm getting better.
I still have an entire wall of books, but at least that's only half of what I used to own!
First, keep the books that really mean something to you.
For me, this was my first games I owned as a kid (Rifts, Vampire, GURPS, Earthdawn). Also, any system I ran a long campaign in is enough reason to hold onto it.
Second, spot the books that absolutely replace some of your other ones; keep those, donate/sell the rest.
For me, those were newer editions that were better than the old ones in every way (Savage Worlds, for example) or games so good at their genre, I would always choose them over others (Interface Zero over Shadow Run for instance).
Third, decide if you really need the "non-core" books for your older games. If you're just keeping the game around for inspiration and good memories, maybe the core books are all you need.
For me, this trimmed down my collection considerably. 3E and 4E D&D in particular are just fine without the extras. I did keep my 3E Campaign Settings though, cause those are just on another level.
Finally, if a game has never seen play and it's been over a year since you even picked it up to read or appreciate the art, get rid of it.
Moving forwards, I recommend getting a large tablet and start collecting RPGs in PDF form. Not only are they considerably cheaper, but the space saved will amaze you.
I find myself still buying the core books of games I constantly am playing/reading, but almost never buy sourcebooks or adventures.
Anyways, good luck!