Once you begin to understand that body and mind are instruments to the soul, you begin to see that the unity of soul and mind does not refer to equal strength in decision making, but to accept the roles each have to play and- as a result- function thereby with confidence. The mind is not supposed to make any decisions. It is meant to receive and interpret reality to the soul and in return understand the direction of the soul and express it through proper interpretation of it. The mind is the first instrument, which in turn uses the body as instrument and even there it goes both ways, since the body utilizes the mind to do actions on its behalf.
The stream therefore goes something like this: soul<>mind<>body
Unity between all of those elements means that each accepts its role and does not claim false dominance. The soul has to accept that its first duty is to function in the physical world, which turns the soul into the instrument of God, or how ever you prefer to recognize the purpose giver, creator, ultimate receiver, eternal entity. I'm somehow sensing that I skip a few forces in between, but for now this should be enough.
That might add this portion to the stream of human existence: Infinite<>Soul<>mind<>body
Anyway, the core issue is that the ultimate unity turns the mind into an alert listening device that cares consciously to read the soul's command. We like to call the soul the heart, but the heart is merely the only way we distinctly sense the instruction of the soul. It makes, however, an excellent indicator for the origin of our inner direction. But it's important to recognize that the heart reflects a greater range of emotions, which may have their origin elsewhere at times.
Reason is important not to come to conclusions, but to properly interpret the physical reality so that it can be understood by us. Actions driven through reason only become truly useful, once we recognize the origin of the actions as being of the soul. Equally reason is meant to formulate the action requested by the soul.
Some people tend to favor their mind and skip their heart. Others even favor the body and skip both mind and soul. Only full unity can provide truly satisfying action and through this the real lesson for the soul to continue evolving. I believe that excuses are devices not just from one person to another, but very much internally a way of justifying a failure to reach unity.
Release yourself from former mistakes! Forgive without excuses! Embrace full unity of your faculties; body, mind and Soul! Know that all of these components make "You", not just the soul, just the mind or just the body. Without any of these components it is not you, but a part of you. To become the real and full you, you must strive for this unity!
It does not save you from making mistakes or does not turn you into a saint. It simply gives you the means to fully function to the best of your abilities and will give you confidence to be dutiful in your will.