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.
The title you recommended to the OP doesn't even make much sense when compared to what the Op used... There are professional gamblers. In case you don't know, it might come as a surprise to you, but some people actually take gambling as their source of income and don't just go there for the sake of having fun.
For professionals, they spend time devising strategies and methods on how to gain an edge over the house. Have you ever sat and wondered how those strategies in gambling came about? Do you think someone who just wants to gamble or have fun would spend their time devising such a complicated strategy? It is usually the professionals who come up with those strategies (not all of them though).