SB 11.29.2
prayasah pundarikaksa
yuñjanto yogino manah
visidanty asamadhanan
mano-nigraha-karsitah
Translation:
O lotus-eyed Lord, generally those yogis who try to steady the mind experience frustration because of their inability to perfect the state of trance. Thus they weary in their attempt to bring the mind under control.
Purport:
Without the shelter of the Supreme Lord, a yogi easily becomes discouraged in the difficult task of fixing his mind on the Supreme.