Sometimes it is simply just losing contact and growing apart which in some ways is a lot better and easier to cope with than the ways I've lost friends in the past. Arguments which spiral out of control also lead to this sad event.
People cope with these situations in many different ways, some cry, some take it out on others, some block it out, and some get over it or may never even get over it. There's one thing I know for sure about loss of a friendship which I think the majority of people will agree with me about, and that is that it hurts so much to lose someone you told everything too and thought you were going to be those 2 people who everyone asks how they remained friends for so long. Then you realize you will be the person asking another couple of friends that question in years to come.
Personally I think losing a best friend is exactly the same as breaking up with a boyfriend or girlfriend because you are essentially losing a part of who you are. From personal experience it didn't come easy for me to accept that this was the reality that I chose to take and even though I knew it would eventually be better for me and everyone around me was telling me that I was better off and that the friendship was not a good one and that I was treated badly, I didn't see that till after it ended. I've taken a long time to accept that my friendship of 6 years with someone i considered my best friend during those years to be over and i'm finally able to speak about it with my loved ones without breaking down completely and wanting everything to go back to how they were no matter how horrible it seemed from the outside to people.
No one ever likes change, especially if that change is one that changed you in the first place. Finding your feet, getting yourself together, accepting the situation is hard. You realize your real friends and family during those times of hardship. Listening to a whole playlist of sad songs, deleting every single picture of you and your friend on Facebook, instagram, every tweet, every text, the number, Skype anything which would be too hard for you to see is deleted and every-time you delete something, you die a little inside. And people say they understand, but do they? No one ever truly understands you, but they try and that is what counts.
My advice if anything is going through something like this currently, do whatever makes you feel better. Don't let anyone tell you that you should be over something if you are not. It's okay to be sad it's okay to want to turn back times but you need to learn to either accept the facts, or do something about the situation if you are not happy with what happened with you and your friend. If it's for the best then leave it, if there is a chance of becoming friends again then go for it reach for it and get your friend back. But always remember, you are your happiest when those around you are what is making you happy. If you are not happy and have grievances with that person. Leave it alone before it becomes a lot worse. Being happy is contagious, take your happiness and spread it like a wild fire to everyone around you.