If you do not believe, it is because you are not trustworthy (Isaiah 7:9).
In these few pungent words, the Prophet explains why people may have difficulty in believing in God: they are not themselves trustworthy. In other words, if I am reliable, and I know that people can trust that I will keep my word as best I can to perform and deliver, then I will have little difficulty in having trust in God and in His ability to perform and deliver. Lack of trust in God is thus a reflection of one's own lack of trustworthiness.
Projection, a psychological defense mechanism, consists of attributing to others those attitudes and motives that we ourselves harbor. The Talmud summarizes this concept in the dictum that those who find defects in others must themselves be defective in the same way. Isaiah is simply applying this principle to trust and faith.
Some people struggle with faith and therefore consult various philosophic works on the subject. The authoritative works - such as Duties of the Heart by Rabbeinu Bachaye, the Kuzari, and others - certainly deserve study, but while they define very well various aspects of faith, they cannot be expected to have much impact on someone who lacks the basic capacity to trust, because of his or her own lack of trustworthiness. One can fine-tune a radio, but the dial will not do much without an electric current.