A difficult note you pose there. It's hard to really define success. As human beings we are never satisfied. We reach one goal, but then want another thus locking ourselves in a constant battle to fill our never ending appetite.
I myself am still new, and have been prone to guilt trips when my fantasy goals are not met. I personally wish to have more subscribers and fans. But if there is anything I have learned, it is to be grateful for what you have. Fans are not an obligation, they are people who really like your stuff and are willing to give you time because they like your stuff.
So what I'm saying is, success is being thankful for what you have because there will always be someone with less. And don't let the numbers get to you if you enjoy doing what you do. And remember it is human nature to always want something else, it is a trait that we must all learn to overcome