It's impossible for anyone to be a professional gambler by winning more than his lost because gambling is a game of luck and you only win by chance. Therefore, the house hedge always win. For this reason, a successful gambler is that gambler who can limit his losses at all cost and have self control over his gambling activities in order for him to be a healthy gambler.

OP, the title should be "what's being successful to you" because we don't have a professional gambler but we can rate our success in gambling based on how much we have won or how discipline we are in our gambling activities.