One especially useful lesson is that one can be deeply involved in one’s spiritual practice for a lifetime, even as a monk, and have progressed far along the path, and still not be completely free of the ego thinking-mind’s grasp. This is not a reflection on the dedication of the practitioner, but on the power of the ego. Until one reaches full enlightenment, its power remains strong.
The other lesson is that the one thing that cannot be taught is enlightenment. It must come from within. One can make great strides in walking the path. One can approach enlightenment. But as to the ultimate point ... total surrender of your ego to your true Buddha nature and full enlightenment ... that can only come from within. All the teaching in the world will not get you to that point.
Since as practicing Buddhists we work towards not having goals or attachments (hopefully, especially not relating to our spiritual life), these lessons should cause us no frustration. It’s just the way it is.
All we can do is apply ourselves as best we can to our spiritual practice, going deeper into the teachings and ourselves, thus slowly internalizing the teachings of the Buddha and turning our will and our life over to the care of our true Buddha nature. When our knowledge of the teachings impacts how we do what we do, how we see ourselves and the world around us, when it becomes part of our being, then we begin to approach enlightenment. And for that we should be grateful.