The reason why you have these alternating moods is because there are two different elements in you. On one side, there is trying to develop in you your psychic being which, when it awakes, gives you the sense of closeness or union with the Mother and the feeling of Ananda; on the other, there is your old vital nature, restless and full of desires and, because of this restlessness and desire, unhappy. It is this old vital nature, which you were accepting and indulging, that made you go wrong and stood in the way of your progress. It is when the desire and restlessness of the vital are rejected that the psychic in you comes forward and then the vital itself changes and feels full of the joy and the nearness. When the old unhappy and restless vital comes up again, you feel yourself unfit, without pleasure in anything. What you have to do when this returns is not to accept it, to call in the Mother’s nearness again and let the psychic being grow in you. If you do that persistently, rejecting restlessness and desire, the vital part of you will change and become fit for the sadhana.
Ref : Letters on Yoga Vol. III