The trick, however, is that you definitely feel confident once you're successful, so you believe (after the fact) that this might be the cause.
Becoming successful by mental affirmations is like saying that you can have a convertible car by just feeling the wind in your face. Owners of a convertible will tell you that this is the real feeling -- but it is not the cause for them having the car in the first place.
The link between mental affirmations and making money is even weaker: many times you don't even need to be successful to have a lot of it. Most fortunes are result of inheritance, marriage, blind luck (lottery winners), being in the right place at the right time (e.g., an early engineer at Google), and, don't forget about it, corruption.