Listen to the people who love you. Believe that they are worth living for even when you don't believe it. Seek out the memories depression takes away and project them into the future. Be brave; be strong; take your pills. Exercise because it's good for you even if every step weighs a thousand pounds. Eat when food itself disgusts you. Reason with yourself when you have lost your reason.
When you're lost in those woods, it sometimes takes you a while to realize that you are lost. For the longest time, you can convince yourself that you've just wandered off the path, that you'll find your way back to the trailhead any moment now. Then night falls again and again, and you still have no idea where you are, and it's time to admit that you have bewildered yourself so far off the path that you don't even know from which direction the sun rises anymore.