As I grow up, I am learning that not all friendships last a lifetime. There are a few that will and praise God for those! However, sometimes friendships are great, true friendships, but aren't meant to last forever. There are certain people in your life at certain times for certain reasons. I am not saying you shouldn't try to keep up with people or continue to foster relationships, because it takes quite a bit of effort to have ANY friends in the adult world! But growing up sometimes means growing apart, and that is ok. It doesn't make the times you shared together any less valuable.
Maybe a tribe is important, but it doesn't mean you have one "tribe" for your entire life, or that a tribe has to be a group of 3 or more women doing yoga together. I think the point is having one or more people in your life who you can be completely real with, no hiding or holding back. Right now, that person is my mom, and I think that is perfectly acceptable.
I'm not writing this to bash the strong groups of friends that I know exist. If you have that- I am sincerely happy for you. I am writing it for those, like me, who have struggled to find that sense of belonging.