I've always been a packrat and it's hard to change a person's mindset but not impossible. It won't be easy to get past the mentality of keeping something "in case" but the clutter is, well, bad. Really bad.
Simpliest way to declutter must be to just throw out everything you don't immediately answer "yes" to in response to this question:
"Do I ______ this?" (read/wear/use, etc.)
Gah, too much stuff!