atha cet tvam ahaṅkārān
na śroṣyasi vinaṅkṣyasi
If you become conscious of Me, you will pass over all the obstacles of conditioned life by My grace. If, however, you do not work in such consciousness but act through false ego, not hearing Me, you will be lost. (Bhagavad-gita 18.58)
One way is to work as a servant doing his small part by carrying out the order of the Supreme Absolute Being for a supreme cause of good for all as ascertained by the supreme intelligence of the Supreme Being. Mat cittah. And the result of such consciousness is that, every obstacle faced (sarva-durgāṇi) in the course of achieving that cause will be mystically crossed over (tariṣyasi) by the grace (mat-prasādāt) of the Supreme Being.
The other way is to work in consciousness that one is himself an independent controller or manipulator of material energy forgetting God. He does not recognise the existence of the Supreme controller controlling this world for a supreme universal order in this world in accordance with His universal laws. Such a person works for an independent selfish desire of selfish enjoyment thinking that the world is meant to be exploited for one’s own selfish or extended selfish pleasure of his relatives. This is what it means by atha cet tvam ahaṅkārān – on the other hand if you act out of false ego.
Arjuna's hesitation to fight out of his not feeling good about killing his relatives (extended selfishness) in the course of the righteous war. He lost sight of the universal picture to establish dharma as explained by Krishna but was rather sentimental about not killing them and refused to fight in the beginning.
Krishna's advise to Arjuna was to fight in his role and his duty as a Kshatriya. By the time Krishna speaks this verse in the 18th chapter, Krishna had already shown His Vishwarupa or universal form to Arjuna. Following the display of Vishwarupa, Arjuna declared to Krishna that you are that Supreme Being. Therefore Krishna is talking about mac-cittaḥ meaning be conscious of Me as the Supreme Being and fight. mām anusmara yudhya ca (Bhagavad-gita – 8.7) meaning remember Me and fight.