“Better is one’s own duty, though imperfect, than another’s duty well performed (śreyān svadharmo viguṇaḥ para-dharmāt sv-anuṣṭhitāt)” — the great refrain, echoing 3.35. But your question is exactly the right one: does this counsel a fatalistic surrender to whatever tendencies one happens to have? No — and the very next line guards against it: “one incurs no sin doing the duty ordained by one’s own nature (svabhāva-niyataṃ karma)” (18.47), and “one should not abandon the work suited to one’s nature, though it be attended with fault” (18.48). The point is not “indulge every impulse” — that is the demonic kāma-kārataḥ of 16.23, expressly condemned. The point is authenticity of vocation: work aligned with one’s true nature (svabhāva), done as duty and offering, ripens one; work borrowed from another’s nature, however outwardly impressive, breeds inner friction and eventual grief. Svabhāva here means one’s deep constitution, discerned and disciplined — not one’s passing cravings, indulged. The verse asks for fidelity to one’s genuine calling, not licence for one’s appetites.