Have you ever heard the quote "It is crazy when someone you are best friends with suddenly becomes a stranger"?, Well unfortunately it happens and it can be good and bad. Now I will clarify myself because that sounded terrible. What I meant was that friendships change as we get older, we find those who we know will stay in our lives and then watch those who go off on their own. Friendships are constantly changing, just like we are. Over time, as we get older we learn who will fight to keep our relationship and some move on. It is kind of like a breakup, but kind of worse.
There are many people who have had their best friend since they were practically born (I don't know how). That kind of relationship lasts years and years, but I have heard stories of friends growing apart, not because they lose interest but they change and find others they connect with more. Since everything around us changes, it is hard to assume that we will stay best friends with the same person through out our entire lives. This isn't a bad thing, it just means that we discover ourselves more as the years pass and find that we don't have as much in common as we thought. Sometimes we find that the people we could get an A+ on a Buzzfeed friendship quiz aren't exactly the person we thought we knew.
As Doctor Suess once said, "Don't cry because it's over, Smile because it happened". That is mainly directed toward break ups, but still, it's pretty good. Friendships may not be broken up entirely, but the "best" part isn't really there anymore. This doesn't mean you have to cry and say it's over I have to find new people...no. In fact, it's not really something to cry about in the first place. Just because your "best" friendship doesn't include the "best" doesn't mean you can't acknowledge each other. It just means that you both realized that you were slightly wrong about knowing everything about each other.
Don't blame yourself, please. Never tell yourself "you should have fought harder to keep them" because I truly believe everything happens for a reason. It sucks, but don't put it all on you just because you learned that things just didn't work out. Don't be angry at them either, it is absolutely no one's fault in this. You are young, you probably had a friendship that broke up too, then you found this one, and now you will find others. Think of it is kind of like a romantic relationship breakup, it hurts and you probably want to kill the next person they snapchat with, but all you have to do is move on and focus on yourself.
