I appreciate your honesty; I suspect many readers who've gone through a similar crisis have had these feelings. I think of Elijah in the Bible, who was convinced he was a failure and asked God to let him die. He declared, "I have had enough, Lord.... Take my life" (1 Kings 19:4).
But God didn't answer Elijah's plea. Instead, He turned Elijah's attention away from his problems, and turned it instead to God and His plans for the future. Elijah came to realize that although his problems were great, God was even greater, and He would give him the courage to overcome his problems.
And this is what I pray you will come to understand. I don't know why God allowed this to happen, but I do know this: God has not abandoned you. He loves you and is with you (even if you find it hard to believe right now), and He wants to help you. The Bible says, "'For I know the plans I have for you,' declares the Lord, 'plans to prosper you and not to harm you, plans to give you hope and a future'" (Jeremiah 29:11).
You can't change the past -- but with God's help you can change the future. Begin by putting your life into Christ's hands. Then ask Him to help you overcome your fears about the future, and to give you patience and courage to discover His plan for the rest of your life.