A long time ago, I heard Joyce Meyer say that when you want to believe but you don't know how just say, "I trust you Lord, help me to trust you."
I think Psalm 13 is a little bit like that. When David wrote, "Will you forget me forever," he obviously was going through a time that he felt distance from God. Things weren't going well, and it just didn't seem like God cared.
Only a few verses later though, he said "I have trusted in your steadfast love; my heart shall rejoice in your salvation."
It's a little bit like, why aren't you there Lord? Yeah, I know you're there, but sometimes it just doesn't feel like it.
I think we all get that way every now and again. We start going through troublesome times, and we immediately feel like God has forgotten all about us. Our enemies are winning, and we can't do anything about it.
But if you just stop and remember everything you've read about God, you realize that he hasn't left. He is always there to carry us through tough times. It's OK to ask God where he is. He'll always tell you he's right beside you.