Mental health issues are one part of the greater human struggle to live well within the world. If you believe in God (and I do), it might be helpful to think of this not as God making a mistake, but as evidence that the world is imperfect. We're only human. We aren't God, so we are going to experience struggle of various kinds. There is nothing wrong with you as a human being. But we as humans do live in a fallen world.
You are right, God made no mistakes when he created you; you are his son or daughter and he loves you greatly! But sin entered the world and then we were all forced to live with pain and suffering. But I believe that God could heal as at any time, and that means he can heal you too, as long as you trust him and have faith that he can. But God performs miracles in marvellous ways, sometimes some people may feel a whole body healing from mental or physical illness or whatever it may be but for others they feel small signs that God is working within them, like better days or reassurance from God that he sees you in this struggle. God does see you in this struggle and is hurt by the knowledge that you are suffering, but the things that God teaches you through a trial about anything from the depths of who you are to the depths of who he is, is pretty miraculous; and even through the struggle you may feel like nobody is there of you have gained nothing but when you look back on it, I promise you will see God's work in your life and how much you have grown with him. Please feel free to message me anytime if you ever want to talk more about this, I am always happy to try and help. :)