Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 2
Why should you suppose it [the effort of sadhana] is vain? The purification of the vital takes a long time because until all the parts are free, none is quite free and because they use a multitude 
of movements which have to be changed or enlightened,– and moreover there is a great habit of persistence and resistance in the habitual movements of the nature. One therefore easily thinks that one has made no progress,– but all sincere and sustained effort of purification has its result and after a time the progress made will become evident.