One of my favorite prayers is the ancient mantra, Jesus Christ, Son of David, have mercy on me. The important thing about short prayers is they are like drinking concentrated orange juice. They may be too compact and strong by themselves. They need the water of repetition to make an impact. Like layering Kleenex tissues together and asking someone to try to tear them, repetition provides a mantra-like atmosphere for prayer and may lead to a deeper level of contemplation.
Try this out. Sit in silence (I prefer to do this while driving long distances across town) and say the following over and over. My Jesus, have mercy. Do this over and over, without interruption, stopping. The mantra element of this short prayer means you may take ten or even fifteen minutes of reciting it without interruption. It you do find yourself thinking about something else, you must begin again. Saying a mantra is not easy. I have yet to master it fully, but I give it the old Hoosier try again and again, like shooting hoops in my back yard.
A wonderful book entitled Monastic Practices by Charles Cummings OSCO, has a chapter on Short Prayers, one of which is the Jesus Prayer, popular as a mantra from the Middle Ages, Lord Jesus Christ, Son of God, have mercy on me, a sinner. Father Cummings also writes about Centering Prayer, popularized by the Medieval text, Cloud of the Unknowing, and also Buddhist prayers, as well as the place of short prayers as means to move from self to God..You should read the whole document, Cloud of the Unknowing, a document about mysticism, but be warned it is lengthy and assumes ideas not always expressed in the text. https://www.catholicspiritualdirection.org/cloudunknowing.pdf
The short prayers in Catholic spiritual direction are important ways to tune up the mind to receive Christ is our hearts.
Praise be to the Father and to the Son and to the Holy Spirit, now and forever. The God who is, who was, and who is to come at the end of the Ages. Amen and Amen –Cistercian Doxology.