There's good news... First tho', I happened to see your msg on my guy's puter and felt compelled to reply. It took til middle age before somebody finally identified my lifelong chronic low-grade depression which absolutely has a genetic, body chemistry basis. So there's no judging here, even if yours might be "just the blues." For me, learning that my 'craziness' had a treatable cause was a relief that it wasn't my fault that I couldn't "pull myself up by my bootstraps."
The good news is there are all kinds of methods to help, many already mentioned. Tho I hate the idea of exercise, finding some chore that allows physical destruction is always helpful -- like whacking weeds, crunching recyclables, etc.
The absolutely most helpful aid for lifelong good mental health is to trade our natural emphasis on the negative to a focus on the positive. When something bad happens, we just hafta repeat the gory details to everybody which builds up even bigger. Instead we need to put more emphasis on good stuff. Oprah calls it a gratitude journal, and a career counselor advised me to keep a daily journal of the good things that happen each day, even if sometimes the good thing is only that nothing bad happened. The key is that when you need to think of what to write down, your perception is altered because you're always looking for good things. And when you look for it, you find it!
Above all else, if the self-help techniques are effective enuf, do talk to a doctor. There are great meds that can help on the short or long term -- being 'crazy' isn't as bad as we used to think.
So now, have you thought of a 'good thing' yet (besides this msg)? There's lots of help just waiting for you to ask, just like you've already started. Remember to treat yourself as you would your best friend!
K8X