GOD for His own reasons allows us to have afflictions which show His strengths through our weaknesses. Adversity being His tempering fire.
Mental health issues vary as to cause. Some can be traumatic events like PTSD or phobias. Some can derive from brain chemistry being deprived or saturated with too much or too little a chemical. It can come from neurotransmitter disorders. It can even come from a Urinary Tract Infection or Inner Ear Infection.
It's not the persons sins, lack of faith, or GOD is angry or displeased with the person etc. Bipolar can come from genetics and can be passed down, it can also come from substance abuse which is becoming more common.
Most mental illness except usually for the far more serious types are secondary {symptoms actually} of a primary physical disorder. You go tell a Shrink that Inner Ear Disorders can give you Anxiety and he will say you're nuts. You go tell an older General Practitioner the same thing and he'll likely say Yeap seen it a few times. And of course there is misdiagnoses all together.
Yes Jesus Christ can heal. However if healing is not granted the person afflicted should not be made to feel it was their fault a bearing guilt they should not be carrying. Remember Paul's affliction he prayed to be taken away? No one knows for certain what it was maybe partial blindness since many of his letters were written by persons he dictated them too. In that respect one passage does point to that as he talks about his own handwriting. Paul was an educated man.
[T]hey brought to him many that were possessed with devils: and he cast out the spirits with his word, and healed all that were sick: That it might be fulfilled which was spoken by Isaiah the prophet, saying, Himself took our infirmities, and bare our sicknesses (Matt 8:16-17).
Jesus is the same yesterday, today and forever. Healing was "granted" 2000 years ago on the cross of Jesus Christ where Jesus "took our infirmities and bore our sicknesses." He healed all who came to him then and he heals them today. Jesus' life here demonstrated that physical or mental healing is not just an "oh by the way" thing or some kind of exception. He deeply cares about each and every one of us who is afflicted and died to take away those afflictions. He is not only in the business of salvation but also healing and restoration, spirit, soul, and body. He who can receive it let him receive it.
God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil (Acts 10:38).
[He] healed all that were sick (Matt 8:16).
There is no record of Jesus ever turning down anyone who came to him for healing, but "he healed them all." And Jesus never afflicted anyone. He always heals and restores those who come to him.
And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people.[A]nd they brought unto him all sick people that were taken with divers diseases and torments, and those which were possessed with devils, and those which were lunatick, and those that had the palsy; and he healed them (Matt 4:24).
And Jesus went about all the cities and villages, teaching in their synagogues, and preaching the gospel of the kingdom, and healing every sickness and every disease among the people. But when he saw the multitudes, he was moved with compassion on them, because they fainted, and were scattered abroad, as sheep having no shepherd (Matt 9:35-36).
Jesus heals EVERY and ALL diseases and he does so, as it says here, because he has compassion on us. Disease is an intruder, a robber that has committed breaking and entering into a person's life. But Jesus has come to kick the robber out and restore abundant life to people because he loves them and has already paid the price for their diseases.
[A]ll they that had any sick with divers diseases brought them unto him; and he laid his hands on every one of them, and healed them (Luke 4:40).
And besought him that they might only touch the hem of his garment: and as many as touched were made perfectly whole (Matt 14:36).
By God's grace, love, and compassion, Jesus lays his hands on us and sometimes by God's grace, we must find a way to touch Jesus, because all who touch him are made perfectly whole.