It's all part of growing up. You'll meet new people in your life and sometimes the old friends fade in and out. The same is happening for your friends too. Don't read too much into it, just choose to be happy.
I've had friends come and go some of which are mean and some of which are immature. I've learned to be more independent and entertain myself. those friends you mention if they are mean and exclude you they aren't real friends. you deserve better, more nicer kind-hearted friends.