'Et in meditatione mea exardescit ignis. And in my meditation a fire shall flame out.' That is why you go to pray: to become a bonfire, a living flame giving heat and light.
So, when you are not able to go on, when you feel that your fire is dying out, if you cannot throw on it sweet smelling logs, throw on the branches and twigs of short vocal prayers and ejaculations, to keep the bonfire burning. And you will not have wasted your time.