The Personal Force is the manifestation of The Force that effects each individual differently. A Jedi’s training must, of necessity, begin in learning the Personal Force, for this is the manifestation which defines the Jedi’s interactions with everything.
The Living Force and the Unifying Force are universal, but the Personal Force is different for everyone. We begin inside, focusing on ourselves and from there branch outward to the Living Force and eventually the Unifying Force.
The Personal Force is like an overlay, a filter which we all use to connect to existence – to all that is.
When out knowledge of the Personal Force is strong, we can connect to other people and things. Our connection to all that it becomes stronger. When out Personal Force is weak, we retreat into a world which is less and less connected, a world unique unto ourselves. Some call it madness. All but the most catatonic have some amount of Personal Force connection. Even those in a coma have some connection. A strong Personal Force connection will help the Jedi maintain sanity in the face of seemingly insane observations.
Without that first step to connect to the Personal Force, there is no connection. With it, the Jedi becomes tuned in, so to speak, with his environment and those he shares it with. The Personal Force is the connection between the “me” and the “not me”.