Paul *asked*, it is not demanded. You will not necessarily be granted the gift of understanding. What you *must* do is to trust, have faith. And respect and veneration for those who have been granted the gift of understanding, both unknown and known (the Fathers of the Church and Fathers of the Desert).
Each Christian will receive one or some gifts, but not all. As the Apostle wrote:
And God hath set some in the church, first apostles, secondarily prophets, thirdly teachers, after that miracles, then gifts of healings, helps, governments, diversities of tongues. {diversities: or, kinds}
{Are} all apostles? {are} all prophets? {are} all teachers? {are} all workers of miracles? {workers...: or, powers?}
Have all the gifts of healing? do all speak with tongues? do all interpret?
But covet earnestly the best gifts: and yet shew I unto you a more excellent way.
1 Cor 12:28-31
There is a more excellent way than of those gifts. Don't be too concerned about them.
First, it is a *gift*. God has gifts for everyone but necessarily the gifts we would like to have. St. Seraphim of Sarov could see the uncreated light and he reports how his mother always prayed to be able to see the light of God and was never granted the gift *for her own benefit*.
Secondly, you will know if God wants you to have or not only if you ask.
If He never grants it to you, don't despair, just ask Him *what* gift has He reserved for you.
Finally, to learn more about it, start with the book "The Way of the Pilgrim". I don't know which edition in English is best, so I ask my friends in the forum to help on this one. Just remember: reading this book is not a contract that binds God to give you the unceasing prayer, but if it is meant to be part of your life, than it will help.
To sum it up in few words: pray, fast, love.
How can I have the gift of 'Incessant Prayer of the Heart'?
1Cor 14:
15.I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.
Paul asked us to pray with the spirit and also the understandings. If God do not give me or grant me to have' gift of tongue/ prayer of the heart' , How can I pray with the spirit and also pray with the understanding ?