Krishna talked about establishing in oneness whatever may be the mode of living (6.30) to attain infinite bliss. There are three major issues faced by us to attain oneness. One is that it is called by different names in different cultures and to compound the complexity, the paths prescribed by these cultures appear to be opposing one another. Secondly, our mind is trained to divide which prevents attaining oneness. Thirdly, we tend to reject what we don't know and oneness is completely new territory for us. Reflecting on these difficulties Arjun asks how to control the mind.
Krishna says, "No doubt, the mind is restless and difficult to control, but it can be controlled by practice (abhyaas) and dispassion (vairaagyam) (6.35). Take my word that Yoga is hard to attain by the ungoverned man but it can be attained by means of striving" (6.36). Krishna had earlier advised regular practice with the determination (6.23) to bring the restless mind under control (6.26).
Vairaagya is the polar opposite of raag or attachment. Life gives moments of both raag and vairaagya on a daily basis but our mind practises only raag which is chasing desires. For example, we can get frustrated in a relationship and when it happens we blame our partner and look for a new relationship instead of realising that a relationship (raag) itself holds the potential for frustration (vairaagya) . The practice of vairaagya is nothing but a deepening of the realisation that we can't attain joy from outside or from anyone. Our past experiences of vairaagya can help us sink in this realisation especially when they are repeated in the present moment
Death is eternal, powerful and a master of equanimity. Many cultures use this to attain oneness by controlling the mind as it is the ultimate vairaagya we can imagine.