Mental soundness does not mean that the mind has stopped developing or healing. It means it has come to a point where it can be quiet, where it can be at peace. It means thinking in ways that make the life you are pursuing possible.
A sound mind will sometimes feel pain, yet it's the pain of growth, not the pain of needless suffering. Once you've won a sound mind, nothing will ever take its health away again. Not if you learn to guard it from what you let in. God's will for each of us is sound mental health and a capable mind. Even for those of us who are the most mentally sick. We can achieve soundness with effort, perseverance. and the determination to never quit pursuing the life we want.