"..we do not know what to pray for as we ought, but the Spirit Himself makes intercession for us.." Rom. 8:26

"..it is well to pour out your heart before the Lord in words. And when you cannot do that, it is sweet to sit quite still and look up to him, and even as the lilies pour out their fragrance before him who made them, so do you, even without speaking, worship God in that deep adoration that is too eloquent for language, that holy nearness that, because it is so near, dares not utter a sound lest it should break the spell of the divine silence that engirds it. Frost of the mouth but flow of the soul is often a good combination in prayer. It is blessed prayer to lie on your face before God in silence, or to sigh and cry, or moan and wail as the Holy Spirit moves you. All this is prayer, whatever shape it assumes, and it is the sign and token of a true believer's life." -Charles Spurgeon

Praying from the heart is essential. And that can be from a written prayer; from the Word of God; from the understanding I pose in my mind; or from a soul that is heavy, and has no words. The key is from the heart, not just the lips.

"And the Lord said:
“Because this people draw near with their mouth
and honor me with their lips,
while their hearts are far from me,
and their fear of me is a commandment taught by men, therefore, behold, I will again
do wonderful things with this people,
with wonder upon wonder;
and the wisdom of their wise men shall perish,
and the discernment of their discerning men shall be hidden.”" (Isaiah 29:13-14 ESV)

*[Not sure if "wonderful" is the best English word for this context.]