I agree with writing music. Play the fuck out of that bass! And take up guitar; you can 'get after it' more on guitar, IMO. And scream! Growl! Do whatever, but be loud as hell (screaming is just my favorite; nothing like throwing on Emperor - Curse You All Men! and just going off with it). Also, try taking up drums. Practice to black metal. Not only will this give you the chance to bash the shit out of something (that's meant to be bashed), you'll be so tired after trying to keep up those blastbeats after a while, you won't be pissed or depressed anymore. Also, drumming feels very urgent, so if it's a 'longing' or 'yearning' sort of anger or depression, this is great for it. But so's guitar, but guitar's great because you can vocalize more clearly emotions through it, since you can make melodies and harmonies -- and then add your own voice over it, if you will. I also agree with writing lyrics. I don't know if this has even happened with you, but sometimes I'll be feeling a really strong emotion, and I'll sit down and write something, and I'll feel drained afterwards.... or powerful. Sometimes you can write something, and then read it once you've finished, and think, 'What the hell have I just written?' You can get chills. But the same (well, even more so) with writing songs, and listening to what you've done. Music is so primal, it's one of the purest ways of releasing emotions, imo. So yeah, play guitar/bass like a maniac, get behind a drumkit, scream like you're possessed or like your life depends upon it, but PLAY MUSIC. Loudly. Then you'll feel better.
Ryan
P.S. Play music was serious -- loudly wasn't, necessarily. Play however you feel -- loud, quiet, doesn't matter. Just play what you feel, and with urgency.