I can understand being upset about the book; when you get really into a book or movie, it's almost like it's real, and you can become very attached to the characters. I cried at both HPB and OOP, though more at OOP, because I loved that character to death. Then again I'm sort of strange; I cry often at books and such, but hardly ever for real happenings.
I don't know how old you are, but hormones in the preteen and teen years can often cause strange mood swings and feelings that you may not be used to. This is a normal part of growing up, and although it sucks it's something that we all have to deal with.
If, however, as you said, you've been feeling down or not yourself for a long time, and you hardly ever seem to snap out of it, you should do something about it. As Iconoplast said, see a doctor or counselor.
It's good that you said you were feeling very happy just before you started reading, but the feeling of not being yourself all the time probably isn't a good thing (though it may just be a natural result of hormones, as I've said).
Again, I don't know how old you are, but I'll say this just in case:
Substance abuse (drugs, alcohol, tobacco) is not a solution to any problem. They will only make things much, much worse!
Ugh. I sound like my Growth Ed teachers. *gets down off soapbox*
*ahem* In any case, I hope you're able to break out of this and start feeling like yourself again ASAP!
