Keep enough of yourself private that betrayal never receives the whole map.

It is not cynical to keep some things to yourself. Full disclosure to everyone assumes that everyone will remain trustworthy indefinitely, and people change, relationships end, and confidences travel further than intended. Holding some part of yourself back is not a wall; it is a reasonable limit on how much any single person can carry away. It also means a betrayal, if it comes, is survivable.

Notice whether you tend to share everything early as a way of creating closeness quickly. Let depth build over time rather than all at once. Real intimacy is not diminished by pacing, and it is considerably safer.