Our higher aspects
Here is another brief introduction from me to help you get started and better understand Leadbeater's writings.
In addition to our physical body, our personality also includes several subtle bodies: our etheric body, which supplies our physical body with life force; our astral body, which allows us to feel emotions; our mental body, through which we are able to think; and our causal body, which is responsible for more complex thought processes. As we evolve, we can eventually use our astral body consciously as a vehicle for our stays in the astral world, and later even our mental body as a vehicle for the mental world. However, our etheric body cannot be used as an independent vehicle for other planes.
But our personality is not our true essence. Every human being is only a tiny part of a larger entity, which theosophists have called the ego. This refers to the soul of the human being, i.e., the higher self, and not the common meaning of the term today, which refers to a separate part of the personality.
The ego, or our soul, resides on the higher subtle planes. It wants to gain experience on the denser planes of existence, as this helps it to mature and develop, but it cannot descend lower than the causal world. Therefore, it sends a small part of itself down to gain experience there – this part is our personality. Later, it brings this small part back again, along with all its experiences of the lower planes. It repeats this process over and over again, incarnating new personalities until it has absorbed everything that can be experienced and learned on the physical, astral, and mental planes.
In addition to our personality and ego, there is a third aspect of ourselves: the monad, sometimes referred to as the I AM Presence. It is our divine self, or, so to speak, the higher self of our ego. It is completely one with the divine and yet still an individual.

Just as our personality is only a small part of our ego, so the ego is only a small part of our monad, too. And just as we enter the ego with our experiences, one day the ego will also enter the monad. Our individuality will always remain intact, only the sense of separation will fall away, so that we will still feel like ourselves, but expanded to include many other individuals.
So in reality, we are much more than our personality and even much more than our soul. One day, we will once again consciously be monads, sparks eternally connected to the divine flame, and we will realize that despite our individualization, we have never left our unity with God.