Most of the time, I am in the same boat as you, doubt/unbelief. Our tendency is to base our faith on feelings, rather than the facts of God's word. When we don't feel forgiven, then we tend to say to ourselves, "I don't feel like God has forgiven me, because what I have done is so awful." There is a good book I'd recommend, and the very first chapter is on Biblical faith. It can be read online here: Princliples of Spiritual Growth by Miles J. Stanford
The whole book is excellent, but if you read nothing else, devour the first chapter, "FAITH."