I hate to say it, but a hobby. It's such a cliche, ("Feeling blue? Just get a hobby!") but it's true. I started writing after a decade of not writing a word, and it reinvigorated me. A purposeless person is a broken one, and I finally felt a purpose.

Things started flowing out of me, the dark thoughts, the stuff that poisons you and makes you want to give up. Once it was out, it was less crowded in my head, and I felt happier. Or at least less dismal.

I even joined a writing group at my local library (didn't know a soul, it was hard at first) and their support helped me thrive. I recommend a group like that to anyone, introverted or not.