How to Say Goodbye
Coming to the realization and accepting the fact that not everyone is supposed to be in your life is one of the most difficult things I've wrestled within my life. A younger me would swear that friends or certain family members were unbreakable but the reality is not that simple. There are some people who you just shouldn't be around.
The connections we make with other people, whether it be family or friend, is driven by experience and likeness. We are generally drawn to people who see the world the same or share the same interests. It's easy to make the assumption in those times of deep bonding that you'll never be separated. In those times it's hard to imagine ever seeing things differently.
If you are in a healthy environment, I think the natural course of your life does change, however. You're introduced to new information and new views that resonate with you more than those before. You change your mind about how things ought to be and as a result our views about what's important also change. The change slowly trickles down the rocks to the very bottom where our relationships are - the last of the dominos to tip. You notice that they don't see things the same as you and the one thing that brought you together before is actually what drives you apart. It's sad, but it's okay.
Coming to this realization should make you realize that it's likely a good sign if you have different friends than you did 5 years ago. That means you're different - hard to say whether for good or bad but generally I'd wager it's for good. It's a sign of growth. Your ideals or thought process changing indicates that you're mentally challenging the existing rule in your mind and seeking to improve. Losing your friends isn't bad, it's just part of life.
There's no easy way to part with people. It feels odd when you recognize that you're drifting away from those people you swore you'd never live without. This parting doesn't have to be hostile though. A true friend will understand your differences and will wish you the best. It's this mentality that has given me a framework that I have inadvertently adopted: Leave people better than you found them. When you part ways with someone and both parties have materially improved mentally, physically, or spiritually, rest assured both are headed for better things.