This is what Saint Isaac has to say on the relationship between liturgical prayer and personal prayer and reading:
When you encounter sweetness in the verses of your liturgy, your intellect greedily desires to enjoy that sweetness continually. And if you wish to grant the intellect’s request, quickly strive to know the cause of such delight. If you reach it swiftly, as a discerning and not as a blind man, it will not be difficult for you to enjoy it always without hindrance. And what, then, is the active agent which is also called ’cause’? A file, without adding anything to iron, makes it bright and radiant, for such is the essence of iron. How? It removes the rust which has come upon it when it was unused. The same holds true for theoria in relation to the nature of the intellect. Bring your mind to the file and you will find how it shines clearly during your liturgy, emulating the brilliance of the very stars. For the intellect is unable to proceed onward without this action and it lingers when unattended. For this reason I said that during psalmody – but not through the prayer of the heart and reading – the intellect can be held fast only with difficulty. But this is not the case with prayer and reading; with them only small care is needed. Furthermore, their assistance during psalmody is necessary. Indeed without them diligence in psalmody will be found to be in vain. And they, again, at the very time when nourishment is furnished, give support to psalmody and not from any other quarter than from these provisions. Hunger, indeed, knows how to procure food; it will not, however, consent to refrain from eating it.
The Ascetical Homilies of Saint Isaac the Syrian (I, 27), translated by the Holy Transfiguration Monastery, Boston, 1984. 134.