Bhagavad Gita 12.9

But if you cannot fix your mind firmly on Me, then, my beloved friend, try to do so through constant practice.

अथ चित्तं समाधातुं न शक्नोषि मयि स्थिरम्।अभ्यासयोगेन ततो मामिच्छाप्तुं धनञ्जय॥

atha cittaṃ samādhātuṃ na śaknoṣi mayi sthiram|abhyāsayogena tato māmicchāptuṃ dhanañjaya||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).