A real-life example to help in understanding the Trinity.
You are asleep and are having a dream. In this dream you are walking through the forest. You are in control of what you do next, whether you go this way or that way.
But you are also partially awake. You observe your dream-self make decisions and purposively make choices. However, your partially-awake-self is also "you." It functions independently of your dream-self and even wonders what the dream-self will do next.
Even though these two selves are both "you," they are at the same time both autonomous entities, each with a volition of its own.
This is merely an example to illustrate it is within our common experience that even though a person is one, he is also many. This example is not intended to imply that God is asleep and has dreams, but merely to demonstrate we have a common experience which in some ways is as difficult to comprehend as the Trinity.