यदहङ्कारमाश्रित्य न योत्स्य इति मन्यसे।मिथ्यैष व्यवसायस्ते प्रकृतिस्त्वां नियोक्ष्यति॥
yad ahankāram āśritya na yotsya iti manyasemithyaiṣa vyavasāyas te prakṛtis tvāṃ niyokṣyati
Word by word
The verse says
If you take shelter in self-importance and think I will not fight, your resolve is false. Your own nature will set you to it anyway.
In plain words
And now Krishna says the hardest thing he says to Arjuna in the whole book.
Your decision is not real. You think you are refusing. You are not in a position to refuse.
Mithyā eṣa vyavasāyaḥ - this resolve of yours is false. Not wrong. False, as in: it will not hold, whatever you have decided.
Where you've felt this · in failure
Your mother's brother announced at thirty that he was done with the family and would not be going back. He meant it entirely. Eleven years later he is the one organising his mother's care, and nobody, including him, can name the moment the decision reversed. It did not reverse. It was never holding anything up.
Going deeper
Being told that your decision is not real is a strange and uncomfortable thing, and the claim is narrower than it sounds.
It is not that choice is an illusion. Four verses from here Krishna will tell Arjuna to do exactly as he wishes, which would be a cruel joke if nothing were up to him. The claim is about this particular refusal, made in this particular state, by a man whose nature has been formed over a lifetime.
What is being said is that a resolution taken while you are in pieces is not a resolution. It is a symptom. It will last exactly as long as the state that produced it, and then the long grain of what you actually are will reassert itself.
This is worth knowing for your own decisions. The ones made at three in the morning, or in the first hour after an insult, feel like clarity and are made of something else. Nothing in them is wicked; they just do not hold, and you can usually tell which kind you are making at the time.