Defining success is always a problem. It's easy to figure what the world defines as such: money, big house, fast car, party hardy, and so on. And I think, for some people, that is success for them.
It's harder when that isn't success to you. Of course, there is the non-material success where accomplishments are measured by how many yoga stances you can master and your hours of meditation and whether or not you've given everything to the poor and marched for every right that you can march for.
Between those lies a more fulfilling road. That of moderation suggested by Apostle Paul. Of having enough financial success to pay your bills and allow for an occasional luxury and having sufficient peace of mind and compassion to help those you can help and pray for the others. That middle road is, for me, the hardest, because it requires control. It requires thought. You have to behave as an adult even when you're being childish, if that makes sense.
It's a journey that I'm still just starting, looking to the horizon, and putting one foot in front of another. I'm not sure anyone arrives in this life, but the attempt, the journey, is one we should attempt if we want to leave the world better than it was when we arrived.