Release of memory of past lives: The number of persons who can remember their past lives is very small compared with the vast majority, who are so completely bound to the gross sphere of existence that they do not even suspect super-sensible realities. The release of such memories is severely conditioned by the limitations of the brain, as long as consciousness is entangled with the physical body and its brain processes. When consciousness is emancipated from the limitations imposed by the brain, it can recover and reestablish the memories of past lives, which are all stored in the mental body. This involves a degree of detachment and understanding that only the spiritually advanced can have. The memory of past lives can come with full clarity and certainty, even to those who are still crossing the inner planes of consciousness but have not yet become spiritually perfect.
Loss of memory of past lives does not affect progress: The memory of past lives does not come back to a person, except in abnormal and rare cases, unless he is sufficiently advanced from the spiritual point of view. This provision made by the laws of life secures unhampered spiritual evolution of the individualized soul. At first view it might seem that the loss of memory of previous lives is a total loss, but this is far from being so. For most purposes, knowledge about past lives is not at all necessary for the guidance of the onward course of spiritual evolution. Spiritual evolution consists in guiding life in the light of the highest values perceived through intuition, and not in allowing it to be determined by the past. In many cases, even the memory of the present life acts as an obstacle for certain adjustments demanded by the spiritual requirements of the changing situations of life. The problem of emancipation may in a sense be said to be a problem of securing freedom from the past-which, in the case of those who are bound to the wheel of birth and death, inexorably shapes the present life.
-Discourses 7th Ed, p315