Our first steps in prayer nearly always involve learning formula prayers, like the Our Father and the Hail Mary.
While these are very good prayers and should never be abandoned, our spiritual lives need to incorporate thoughtful mental prayer.
This type of prayer is the simple relating of our joys and sorrows to God.
Preference for mental prayer
St. Francis de Sales goes so far as to prefer mental prayer over formula prayers in his Introduction to the Devout Life:
The Rosary is a useful devotion when rightly used, and there are various little books to teach this. It is well, too, to say pious Litanies, and the other vocal prayers appointed for the Hours and found in Manuals of devotion,—but if you have a gift for mental prayer, let that always take the chief place, so that if, having made that, you are hindered by business or any other cause from saying your wonted vocal prayers, do not be disturbed, but rest satisfied with saying the Lord’s Prayer, the Angelic Salutation, and the Creed after your meditation.
He then explains that if you feel drawn to speak plainly to God in mental prayer while reciting a formula prayer, you should not be afraid to stop what you are doing and change course:
If, while saying vocal prayers, your heart feels drawn to mental prayer, do not resist it, but calmly let your mind fall into that channel, without troubling because you have not finished your appointed vocal prayers. The mental prayer you have substituted for them is more acceptable to God, and more profitable to your soul.
One of the primary reasons why mental prayer can be more powerful than formula prayer is that typically this type of prayer involves exposing our heart to God.
It can be tempting to use formula prayer as a crutch, where you never speak to God in your own words.
God desires above all to develop a relationship with us and it is difficult to do that if we never talk to him as if we were talking to a friend.
Formula prayer is certainly a great practice, but if we never speak to God from the depths of our heart, we will likely never grow in our love of God.