Grave (or mortal) sin is any sin consisting of grave matter (i.e. the Ten Commandments) that is committed knowingly and willfully. The *Catechism of the Catholic Church * (CCC) defines mortal sin as: “A grave infraction of the law of God that destroys the divine life in the soul of the sinner (sanctifying grace), constituting a turn away from God. For a sin to be mortal, three conditions must be present: grave matter, full knowledge of the evil of the act, and full consent of the will.”
Regarding prayer, do not be discouraged when your prayers are not answered in the way you expect, but trust that God desires what is best for you. “…our battle has to confront what we experience as failure in prayer: discouragement during periods of dryness; sadness that, because we have “great possessions,” we have not given all to the Lord; disappointment over not being heard according to our own will; wounded pride, stiffened by the indignity that is ours as sinners; our resistance to the idea that prayer is a free and unmerited gift; and so forth. The conclusion is always the same: what good does it do to pray? To overcome these obstacles, we must battle to gain humility, trust, and perseverance.” (CCC 2728)
“Are we convinced that “we do not know how to pray as we ought”? Are we asking God for “what is good for us”? Our Father knows what we need before we ask him, but he awaits our petition because the dignity of his children lies in their freedom. We must pray, then, with his Spirit of freedom, to be able truly to know what he wants.” (CCC 2736)
For more on prayer, see Part Four of the Catechism of the Catholic Church available through Catholic Answers or online at the United States Conference of Catholic Bishops web site.
Also these two books are available through Catholic Answers:
Handbook of Prayers
Prayer Primer: Igniting A Fire Within