Dude, if tweak is messing with your brain, step up off that shit.
Meth is nice if you can incorporate it into your life happily, but it sounds like your brain isn't really feeling that.
Benzos are OK, but realistically they're only any good for acute freakouts, severe anxiety, smoothing a nasty comedown, or to knock yourself out for a good 16-24 hours. You don't want to be trying to use that shit on any regular basis if you ask me, they tend to make me more depressed, not less.
Opiates are definitely an option; they're going to give you endorphins, which is basically a way of cheating up the effects of serious excercise. Realistically, for maximum antidepressant effects you should look into the good SSRIs like Zoloft or maybe Wellbutrin, take opiates when you're really feeling down, and go out and get some hardcore excercise, say an hour long bike ride or something.
You'll feel a helluva lot better than you will on some shitty street tweak, or even high-qual meth for that matter, if you want my opinion. Your mileage WILL vary, of course.
Meth has its place, but it's not for everyone nor for every time/place/purpose. It's sure as shit not on my short list of things that will make your life a happier one when used chronically.