Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 4
These [thoughts of unfitness for Yoga] are the usual suggestions that come when there is the attack of the hostile forces. You should know that they have no value and reject. The spiritual 
perfection, the full transformation of the nature is not a thing that can come without long and steady endeavour. Movements like these have many refuges in different parts of the being and it is not till they are driven out of all and out of the environmental consciousness that one can be free from their recurrence. One must learn to be inflexible, fixed in one’s aim and not discouraged by the recurring difficulties of the nature, for they have been long ingrained in the vital and physical of the human being and they are also in the play of the universal forces – so if it takes long to get them out, as it does even with the best sadhaks, that is no proof that one is not fit for the sadhana. Reject all that and go on steadily – aim always at getting more of the higher consciousness down, that is the cure for all these things.