A positive mindset does not guarantee victory in gambling. However, in my opinion, having a positive mindset is still important because it helps us remain realistic in facing the results, whether winning or losing.
With a healthy mindset, we become more aware of our own limits. We are not easily provoked by emotions when we lose, and are not greedy when we win. This makes us wiser in managing money, and prevents us from spending more than we should. So, even though it cannot guarantee victory, a positive mindset still plays an important role in keeping gambling a controlled activity and not ruining our lives.
I prefer to go with the positive mindset for retaining my belief that winning is possible and go with a healthy mindset to make sure that even as I have that positive mindset, there is still a balance in check on my side, which will not allow me to break my limit. Irrespective of how positive I am, I will still be gambling within limits, not allowing greed to control me and my losses to push me to chase it.