Yes, it is possible to converse with your Higher Self. First you have to honestly believe in the Higher Self, and that the person you think of as your self is not it. At the same time, don’t let the name "Holy Guardian Angel" deceive you into thinking of a separate being that is so holy as to be beyond your ability to deserve the attention of the HGA.
True, the HGA is normally distant from the personality that is the everyday "you," but the function of the Great Work is to build a relationship between the personality and the Higher Self leading towards Integration—when the two become as one.
This practicality is rather unique among magickal oaths! Many are too grand for realization within a single lifetime. While their intention is to stimulate spiritual growth and attainment, it is usually not some grand event or rare or expensive artifact but very ordinary things that call to us for our deeper awareness. Their symbolism may be obscure—normally—but suddenly they glow with meaning or practically yell at your for attention. Or, it may seem like nothing, but still your attention has been re-directed to the event as if it has a special meaning for you, and it does!
But, note further: this oath directs your attention to everyday events of all kinds so that your awareness is alerted to the greater meaning and potential that each may represent. Magick and meaning may be found in the most mundane of events when there is that possibility of relationship between the inner you and the inner side of the event. The effect is to activate connections to your Higher Self, and that is the goal.
"I swear to tell myself the truth." This, of course, is very challenging. What is truth? Can we ever really know it? Again, Culling has given us a good but simple example. It is perhaps too simple. The real requirement is to be honest with yourself; to test your answers for their truth and honesty. This is more than "knowing yourself" for it is also a test of your truth and honesty in relation to others and to the world you live. We too easily deceive ourselves and once again the entire purpose of the oath is to prepare yourself to communicate with your Higher Self, the HGA.
Even though your communication with the HGA isn’t always in plain English, it is not a game! The Great Work is serious business.