The path of liberation starts within. We have potent forces inside, which, if devised wisely. The Gunas belong to the intelligence and not to the self. They subdue each other, which means a guna will only dominate when the two others are suppressed,at the same time they support and produce each other and consort together, meaning they depend on each other and never exist or act separately. Through the use of Sattvik events, thoughts or behaviors, the Sattva Guna is stimulated and strengthened. This is known as Inner transformation of Yoga. 

The psychological consequences of rajas and tamas make the endeavour of controlling the mind impossible; Sattva is the only constituent that saves mind control from being a hopeless task...

Sattva is the highest of all gunas. It is synonymous to purity, knowledge and joy.  Rajas whereas is the principle of aspiration, enthusiasm, longing and restlessness and Tamas is the lowest of all gunas, it leads to inertia (inaction), anger, lifelessness and delusion. When, a reference of a calm composed mind is made, a predominance of Sattva is what is being referred to.

It is extremely difficult to control the mind. Hence no failure should not be taken seriously and no success as final. Failures, instead should be spurs to more determined, sustained and intelligent efforts.  Through continous Practice and Detachment , Mind controlled is possible and every doubt to the contrary must be ignored.

Though the Mind seems to cognize but it is not the cognizer, it is not the source of light even if it seems luminous. It borrows its radiance from the Self (Atman) and acts just as an internal instrument (Antahakaran) for it. It is through mind, the Self i.e. Atman or pure consciousness, that is free and eternal, apprehends and responds to the external world.