Hey man, keep your head up. Keep that light inside you glowing, no matter how dim it might get.
I have a temperament similar to yours, and after several bad years my light has only recently started to glow again. I can't tell you how wonderful it is to feel positive again without feeling stupid, naive, or undeserving. I'm still a long way from where I want to be, but it's a wonderful feeling to feel positive, even if it only is a few moments in a day.
Keep working at it and with regards to having to keep up a happy face for your friends: don't set yourself on fire to keep others warm. Lean on them when you need to. We all need support and love at times