St. Ephrem wrote this in relation to understanding Scripture, but I think it relates to just about any area of the spiritual life:

Be thankful then for what you have received, and do not be saddened at all that such an abundance still remains.  What you have received and attained is your present share, while what is left will be your heritage.  For what you could not take at one time because of your weakness, you will be able to grasp at another if you only persevere.  So do not foolishly try to drain in one draught what cannot be consumed all at once, and do not cease out of faintheartedness from what you will be able to absorb as time goes on.