dhrityaa yayaa dhaarayate manahapraanendriyakriyaahaa |
yogenaavyabhichaarinyaa dhritihi saa paartha saattvikee || 33 ||

That fortitude which is accomplished through yoga, which regulates the activities of the mind, life forces and that senses, such unwavering fortitude is saattvic.
dhrityaa : by fortitude
yayaa : by which
dhaarayate : regulates
manahapraanendriyakriyaahaa : activities of mind, life forces and senses
yogena : accomplished through yoga
avyabhichaarinyaa : unwavering
dhritihi : fortitude
saa : such
paartha : O Paartha
saattvikee : is saattvic
To recap, Shri Krishna classified several components of action into three categories: saattvic, raajasic and taamasic. First we looked at jnyaanam or knowledge, followed by karma or the action itself, the doer or kartaa, and buddhi or intellect. We also saw that jnyaanam gives us a target or a goal to act upon, and the intellect creates a plan to attain the goal. Now fortitude, another aspect of action, is examined in three shlokas, and is classified in the same manner as the other aspects of action.
Fortitude is an essential quality of the intellect which is useful in both material and spiritual endeavours. A seemingly simple action like going to the market to buy fruits requires a certain degree of fortitude. Some people will finish that task within no time. Some people will start heading to the market, but get distracted and go to a restaurant instead. Some other people may not want to get up from the bed, and in doing so, either procrastinate or forget the task entirely.
Shri Krishna says that the fortitude which is able to focus the mind on the task at hand, and also, is able to restrain the mind, energy and senses from straying away from the task, is saattvic. The task will be conducive to liberation, since it has been prompted by sattvic knowledge. The key quality of such fortitude is that it is avyabhichaarini, which means never wandering, never wavering, never flitting from one thing to another. Ultimately, we have to learn how to master our mind by mastering our thoughts.
Now, such immense fortitude can only be accomplished through yoga, which refers to the consistent, repeated practice of keeping our mind engaged in the self. Such strength can only be gained by daily studying of scriptures, daily worship of our deity, maintaining a good diet and so on. If we cannot even restrain our senses for dietary reasons, we will never be able to get to a state where the mind is constantly engaged in the self.