I'm sure many people reading this column will pause to pray for your daughter, for they can sense your deep concern for her as she bears this heavy burden. The Bible commands us to "pray for each other so that you may be healed" (James 5:16).
I strongly urge you seek the best medical advice you possibly can for your daughter, if you haven't already done so. Prayer is important, but sometimes God answers our prayers through other people, including those to whom He has given special gifts and abilities. This includes men and women with scientific and medical skills; one of the Apostle Paul's closest companions was Luke, whom he referred to as "the beloved physician" (Colossians 4:14, NKJV).
I'm not a doctor or psychiatrist, of course, but I understand that depression can have many different causes. Some (such as chemical imbalances in the brain) often respond to treatment, and you should do all you can to be sure your daughter is properly diagnosed. Your pastor or family doctor may be able to suggest the best options in your community.
In addition, let your daughter know that you'll always love her, and that God loves her also, whether she "feels" His love or not. May she also find daily strength in the promises of His Word, the Bible. "Praise be to the Lord, to God our Savior, who daily bears our burdens" (Psalm 68:19).