What should I do when I feel useless to God?
You might first want to ask yourself why you feel useless to God. Maybe you're experiencing some spiritual dryness. If that's the case, take some time to examine your spiritual life. Ask yourself: how's my prayer life? Am I reading good spiritual books? Do I have unconfessed (mortal) sin? Do I frequent Holy Communion? Am I using my time and talents to serve Christ?
If you have a weak or nonexistent prayer life, then pray. Pray no matter how you feel. St. Teresa of Calcutta said, "Everything starts with prayer. Love to pray—feel the need to pray often during the day and take the trouble to pray. If you want to pray better, you must pray more." You don't have to spend hours on your knees every day to be effective in prayer. Setting aside a few minutes of quiet time each day for prayer and reflection is fine. Just pray.
Finally, our battle has to confront what we experience as failure in prayer: discouragement during periods of dryness; sadness . . . 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 . . . To overcome these obstacles, we must battle to gain humility, trust, and perseverance (Catechism 2728).
"When we pray we speak to God; but when we read, God speaks to us." — St. Jerome
Read good spiritual books. If you don't have any spiritual gems on your bookshelves, go to your local Catholic bookstore or online (shop.catholic.com) and do some browsing. A few minutes of spiritual reading a day is good food for the soul. Ask the Holy Spirit for guidance in selecting a book that is best for your needs.
Some books to consider: When God is Silent: Finding Spiritual Peace Amid the Storms of Life by Archbishop Luis M. Martinez, Mother Teresa: Meditations from a Simple Path, or maybe The Life-Changing Power of Doing God's Will by Father Larry Richards.
Unconfessed mortal sin? This can be a huge source of spiritual problems. Go to confession, even if you don't feel like it. Scripture doesn't tell us to seek direction in our feelings. Christ says to seek him, and he who seeks finds, and to him who knocks the door will be opened. Jesus is the door and the way (CCC 2609). The door of the confessional is always open and Jesus waits for you, wanting to forgive (CCC 1496).
Do you receive frequent Communion? The Church encourages the faithful to receive Holy Communion frequently (provided one is in a state of grace), even daily if possible. The spiritual fruits of Holy Communion are numerous: it "increases the communicant's union with the Lord, forgives his venial sins, and preserves him from grave sins" (CCC 1416).
"The only cure for sagging or fainting faith is Communion." — J.R. Tolkien
Are you using your time and talents effectively? Your life has purpose and value (Eph. 2:10). Set aside some time to discern your gifts and ask the Lord how he wants to use you. He will show you the way.