In that case, it sounds very, very similar to what I've learned to deal with -- especially as you describe feeling isolated from the people around you. I started to write a long, long comment, and then realized that I'd probably seen this stuff written down better, somewhere. This matches my experience precisely.

For me, the most important realization was that the feeling of nihilism presents itself as a philosophical position, but is never caused or dispelled by philosophy. You can ruminate forever and find no reason to value anything; philosophical nihilism is fully internally consistent. Or, you can get exercise, and spend some time with friends, and feel better due not to philosophy, but to physiology. (I know this is glib, and that getting exercise when you just don't care about anything isn't exactly easy. The link above discusses this.)

That above post, and Alicorn's sequence on luminosity -- effective self-awareness -- probably lay out the right steps to take, if you'd like to most-effectively avoid these crappy moods.

Moreover, if you'd like to chat more, over skype some time, or via pm, or whatever, I'd be happy to. I'm pretty busy, so there may be high latency, but it sounds like you're dealing with things that are very similar to my own experience, and I've partly learned how to handle this stuff over the past few years.