I don't know about fully functional, but I think there's 3 things I would say are key to not being miserable all the time:

• Be kind. It's not in everyone's nature, but the results are so rewarding. Just stop to think, is what I'm doing right now causing someone else pain or discomfort? How could I reverse that? Don't let your bad day ruin someone else's.
• Be purposeful. Find things to strive for. Small goals are fine, and sometimes things take a long time, but don't lose sight of where you want to be. Dont be manipulative or treat it like a zero sum game either, your success doesn't need to come at the cost of someone else's. Winning with your friends is even better than winning alone.
• Be forgiving. Most importantly to yourself. Failure is not the enemy of success. Self hatred will destroy any chance you have at happiness, many of us are taught at a young age to treat our own failings as horrendous sins that we must mentally self flagellate for. This is the one of the hardest things to overcome, but every step along the way will give your mind a little more room to find peace.

I found myself in a place where I was terribly miserable, isolated, and regretful. I didn't know it at the time but it was the gradual application of the above which helped pull me out of that place.