Is sending a gift (an art painting) to a psychiatrist, after receiving intensive counseling from him, unlawful and a cause for sin, and what is the way to gain God’s forgiveness?

If the doctor is an employee who receives a salary from official bodies, then it is not permissible for you to give him a gift, nor is it permissible for him to accept it, because it falls under the category of forbidden bribery. This is based on the saying of the Prophet, peace and blessings be upon him: "Why did he not sit in the house of his father or the house of his mother and see whether he would be given a gift or not?" However, if he does not receive a salary from public funds for his work and volunteers for you, then there is no objection to rewarding him for his effort and work, based on the saying of the Prophet, peace and blessings be upon him: "Whoever does a favor for you, reward him."