There comes a point in everyone's lives where someone you know, becomes someone you knew. This could relate to friendships, relationships, siblings, parents - really, anybody you were once close with. The saying is cliché, but it's one hundred percent true. It is possible to be so close to someone that you know them like the back of your hand, and to suddenly feel, like you never knew them at all. It's not the greatest feeling when this happens. It's never something you would want to happen - a breakup, a friend break up, divorce, falling out - whatever you might call it, it's definitely not something you want to happen to you and whomever you may be close to.
It seems to me that these sort of things happen when you least expect, and with whom you least expect. My use-to-be BFF's are all now girls I use to know, and although it was painful at the time, I got over it. But that doesn't change the fact that it sucked to go from sleepovers and gossip, to distance and whispers. That person that once claimed you as your "Best Friend Forever," now might be a person you'll never talk to again, or at least, not in the same way.
It might be hard to deal with, and it might be hard to ever trust someone like the person you thought you could trust for your whole life, but you really do just have to move on with your life. Whether it was losing a boyfriend, best friend, or someone else close to you, the best thing to do is just get over it, because those people don't deserve what you have to offer. If they blindsided you, then maybe they were never the best friend or significant other that you thought they were.
Honestly, I've gone through it. I have a couple of friends I thought would be standing next to me in my future wedding, and I'd be lying if I said it didn't hurt when those friendships ended, but it's made me aware and choosey of who I pick as my closest friends. Granted, you cant choose all of your friends, sometimes friendships can bloom out of nothing, but you can choose to be more conscious about if they'll be there for you in the long haul, because by now, you'll have learned that you just can't assume that they will.
While it sucks to have friends, family, or significant others who are now just people you knew, it gives opportunity to fill those places and be more careful about who you're sharing your life with.