Friendships are relationships. Can someone know you completely? Will you never change? Will you never discover something about yourself you previously didn't clearly know? Will you never reveal something to others that previously you hadn't? You are as changing and unpredictable as anyone else. Why do you expect the impossible from other people? They cannot stay static for you. They cannot possibly reveal completely everything about themselves to you. I hope you can work on how to accept people for the changing and unpredictable beings that they are. Are you working on this type of thing in therapy perhaps?

Yes, you probably did know you partner well, but the point is that even the people we know really well will surprise us at times. It is impossible for that to not happen. The question is whether what you discover is simply a new facet of that individual or if it is something about them that you simply cannot approve of. That can happen as I suspect it did with your partner, but that does not mean every new piece of information about a person will always be negative.