That even the people you love the most, people you have known for years and that you trust more than anyone else, can always find a way to break that trust.
This was the hardest life lesson I had to learn. It's not that I didn't know that this was a possibility or that I was living in some sort of illusion that people are perfect and that this won't happen to me, I knew it could, but there is a huge difference between knowing that it can happen and to have it actually happen.
When it happened, it taught me a lot, I understood that even though people can in fact be trusted, this trust cannot be blind trust, it must be maintained and must be nourished and guided. I learned that it's not just about the trust itself, it more than that, it's about maintaining that trust and keeping it at the level it should be, as you need it to be, and understanding what the trust actually is, the strength of it, and, what its breaking points are because everything has a breaking point.
I learned that just as everything has a breaking point, so does trust, and if you do not understand where this breaking point is, at some point, it will break without you realizing and understanding why, it will be sudden and painful. Knowing where the breaking point is will help you be ready for when it actually happens, and, it can help you prevent it from breaking in the first place.