mental illnesses dont exist. There are just people who ve been loved enough, and others who werent. "Illnesses" develop because of sins. Both physical and mental ones.
All you need to build faith is someone who has it and will share it with you. As these "angels", messengers of God are very rare nowadays, you can directly consult God by reading the Bible and start conversing with him everyday until you start purifying yourself.
He left it for us, His creatures, so we can live happy lives accomplishing his mission and using our body and soul in this endeavour, which will give a meaning to our lives when others who worship themselves, material goods, sex and money as their gods will lose the will to live.
We are preparing the kingdom of God on earth.