Definition of mindset obtained from Google Search: “the established set of attitudes held by someone”
Whether you define success as the achievement of an external set of circumstances, such as the achievement of goals, or define it as an internal state such as peace and bliss, both definitions require a mindset which is conductive to them, a mindset harmonious with them.
The reason behind this is, our mindset is fused with how we have come to define ourselves, how we see our reality. Our established mental attitude is a reflection of how we see our reality, how we wish to interact with it.
If our mindset is grounded, is in a non resistant configuration to what is already happening, we can achieve our goals with greater success, because we can see them coming into our lives more easily, and we can more accurately select them with this clarity.
As an example, a scientist who knows how planetary bodies behave based upon studies of generations of observation is much more likely to successfully create (a method of) navigation for a spaceship than a scientist who has limited data on their hands, and an unproven conceptual projection of how these bodies should behave. It can be said that the second scientist is more in resistance (and unaware of it) because they rely more on mental projections and less on facts and direct observation.
Our mindset can become more conductive to achieving our external goals if we become relatively more grounded (gradually) through living life in awareness rather than unproven concepts within our mind.
Likewise, internal states of sustained peace and happiness are also the hallmarks of evolved mindsets. The more evolved a mindset, the more it is based on truth and the less on illusion. And deeper truths are found out through experiential realization of our oneness with reality, the oneness of everything. As this is being realized, the resistance in the mindset is also being released, as it is very natural to act according to an already realized truth. If you know that burning your hand creates pain, and you don’t want pain, you naturally have a desire to avoid burning your hand, if there are no other complicating circumstances. Likewise, as we are realizing the oneness of life, we are also learning to live in flow, in harmony with it and this is also a state of peace and happiness within. It is reported that at very high levels of realization, the inner experience becomes totally peaceful and blissful. (Example: Ramana Maharshi)
If our mindset is directed towards realizing external and internal truths, this will lead our life towards success, because we will naturally want to become aware of facts, through experience, and we will be alert, focused on this, depending on our commitment to this. This mindset can be cultivated if one really desires to go towards this path. The reason this can be cultivated is because, there is an intrinsic desire to experience truth in every human being, and this can be uncovered through naturally aligned action (mental/emotional/physical) and focus.
It is very important to state that no matter what your mindset is, it can be evolved towards outer and inner success, which can not be separated, but were spoken of as if they were separate in the beginning of the answer to give examples for different perspectives. External achievement becomes meaningless without internal peace and happiness, and consistent internal peace and happiness is an experience of a sober mind, in acceptance of what is, therefore is not in resistant desire that things should be different than what they already are. Whereever you are, it is possible to grow/awaken/gradually experience greater success.