Nobody wants to lose friends, for obvious reasons. You create memories with this person, adventure with them, experience new things or experience them in a new light. You tell them your secrets, what makes you you, and they do the same. You get to know this person and they get to know you. You hang out, go to movies together, see your favorite artists perform, drive around aimlessly together. Then, slowly you do not text as much, you hang out but more in a large group setting where you may not even have the chance to say any more than a "hi" and "how have you been?"
There does not have to be a huge fight or a groundbreaking reason that this is how you guys turned out, sometimes, life just happens. Not all friendships are meant to ask the long-haul. This may sound silly, but this may not be a bad thing. Not every friend you have will be with you until the end of time, and that is okay. These are the friends we can learn from, grow from. You will learn about yourself and grow as a person with all your friends, but some are meant to be temporary; they are not meant to be your bff.
Losing a friend is never easy, and sometimes it hurts more than you think. Whether the friendship ended badly or you just grow apart, you can learn a lot from everyone you come across. These friendships can teach you how you deserve to be treated and how you do not, what kind of activities you enjoy and what ones you don't. You can learn to stick up for yourself, to not let anyone treat you in a way that is not okay. You do not need to dramatically cut yourself off from those who do not belong in your life anymore, you can simply distance yourself. At the end of the day, you must do what is best for you and what is best for your well-being.
Never, I mean never, keep someone toxic in your life because of how close you are as friends. No memories or amount of time your friendship has lasted is worth your unhappiness. There is no shame in putting yourself first and removing those in your life that no longer fit. There is no shame in creating an atmosphere for yourself that makes YOU the happiest. If the only reason you are still friends with someone that does not belong in your life anymore or is not making you happy is the time you have been friends, that is not reason enough. Surround yourself with people that you want to be. Take all friendships and all people you encounter in your life as a lesson; keep those who bring out the best of you and separate yourself from those who do not value who you are.