As we get older, we start to drift away from friends that we thought we'd have for life, and it hurts to realize that we won't have those people that we thought were like family anymore.
I've had countless people come into my life and leave without explanation, and I've drifted away from people who I thought would be in my life forever. Situations like those made me fall back from the people who cared about me, and I'd wonder why people kept leaving. I would think that there was something wrong with me and I'd ask myself, "Why do the ones I love most keep leaving?"
I trusted those people and opened up about the deepest parts of me... and they left.
Even though it hurts like hell when people that you've been close to for years leave, nothing is wrong with you. People just simply drift apart. You outgrow people.
Instead of looking at it like it's a bad thing for you, look at it as if you're just simply growing.
You can't keep fighting for those who are trying to grow, even if you don't want to see life without them. You have to let them and yourself grow. Keeping them in your life will just hurt you and keep you from growing into the person you're meant to be.
Trust me, it sucks to trust someone with your life and have them leave one day out of the blue for an unknown reason, but it does get better. Within a few months, you'll be living your best life and you'll know that you're better off without them.
Yes, from time to time, you'll think about them and miss them, but the truth is, you'll get over that pain. You'll be yourself again. It all just takes time.