Please try to interpret my behaviour:

I used to be really emotional and interested in my life, not taking any shit from anyone etc. When I didn't like something/someone, I'd face the problem straight away.
Lately, it is as if I've become less "passionate" about everything and whenever I fight with one of my friends or something else happens that I normally wouldn't have tolerated, I just retreat into a sort of "life is meaningless, so why bother"-mode. Does this make any sense?

Right, so this behaviour freaks me out to no end!! It really does.

Any help?

One more thing: Whenever I argue with someone, it is as if I think the whole thing is ridiculous and so is my opinion.

sounds like depression. not enjoying things (anhedonia), and feelings of unworthiness (you feel your opinions are ridiculous). i don't know if you have other symptoms, but you could be mildly depressed.

or maybe you are just getting old and jaded.

The thing is that I don't like this feeling, so therefore if I was really depressed, I wouldn't even have bothered to write this thread, right?
I think it's partly some kind of self-protection. Lots of things have been bad lately, so I guess it's just natural to block some emotions. Right?

*bump*