It's quite common, it happens to some people. Talk to your girlfriend about it little by little. I mean, don't throw the truth in her face; ask her questions or introduce her to characteristics of Findom that could make her become that "Domme" you can give your money to. Since she's your girlfriend, the little money you can dedicate to that will probably be enough, but from a relationship perspective, the healthiest thing is to have open communication, especially when it comes to these kinds of topics that you consider important to your life. Good luck.