What should I do about the obsessive doubt concerning the existence of God, knowing that it is an obsession, and what is the beneficial remedy to get rid of what I am experiencing?

We ask Allah to grant you healing and well-being. There is no contradiction between reliance on Allah and seeking medical treatment. Therefore, we advise you to consult a doctor while increasing your supplications and reliance on Allah. The best treatment for obsessive thoughts is to ignore them and pay them no mind. They will not harm you as long as you dislike them; indeed, your dislike of them is a sign of the soundness of your faith. Strive to repel them and do not give them any attention, for you are upon goodness and will be rewarded for your struggle against them.