My oldest friend [28/F] has removed me [28/F] from her life without a real explanation.

I've had this friend since high school, let's call her Harriet. We don't hang out a lot because I'm now a mom, but I've always thought we had one of those timeless friendships where we might not see each other for like a year, then meet and catch up like no time had passed at all and everything's cool. Or at least that's how it was in the past.

About a year and a half ago, a mutual friend of ours suggested I add as a friend this account I'd never seen before on Facebook. It was clearly Harriet but she was using a playful alias she sometimes goes by. I figured she was using this account to replace her old one, and requested her as a friend.

She didn't accept my friend request. I don't care about Facebook, but I thought it was weird that she had 100+ friends on this new account and didn't add me, so I was thinking maybe she was hacked.

A month or two later when I saw her in person I mentioned it. She said that she has two accounts, one for acquaintances/the general public and one for her "true friends". I said that she didn't accept my friend request (not in an accusatory way, just as an observation)... She said, verbatim, "Well, I don't always make the best decisions." I thought that that was a weird response and wasn't quite sure what to make of it. I said something like, "We've known each other for so long, you know I love you and I would never judge anything you do, right?" And she reluctantly said, "Well... ok, yeah, I'll add you."

I thought it was totally weird and unexpected. This is the woman who threw me my baby shower and someone I considered a sister. So, I requested her as a friend again, and she doesn't add me again. Again, I don't care about Facebook and hardly use it at all, but I'm bothered about what this is saying about our friendship.

We don't see each other for awhile, then she asks to hang out via text. I probably should have said this in person, but I ended up sending a text saying that I was kind of confused about the whole situation. I said it bothers me because it feels like she doesn't think of me as a real friend. I said I was a bit hurt because I always thought we were close. It was a pretty heartfelt message, and she replied with just, "Sorry that I hurt you. I won't bother you." I thought that was weird, and clarified that I wasn't mad or anything but I really just wanted to figure out what's going on between us because her friendship is important to me. No response.

About a week later she invites me to like her business's page on Facebook-- she's a babysitter-- and I "like" it and say that I would love to have her babysit my son sometime. She sent an enthusiastic reply along the lines of, "Sure, I'd love to, just let me know when."

A few weeks after that she deleted me from Facebook completely, no warning.

A couple weeks ago, my ex passed away from an unknown illness. Harriet was there when my ex had her first heart attack and really supported me. She knew and hung out with my ex many times. So I let her know that she had passed. No response. I then said, "(My ex) passing made me realize just how short life is. I don't know what I did to make you not want to be friends with me anymore, but I'm sorry. I hope we can be friends again someday." She replied with, "I'm sorry about (ex). Take care."

I'm so at a loss for what happened here. We have hung out so little in the past year or so that there isn't anything I could have done or said to make her mad (that I know of). I went back through our messages and just a little over a year ago she thanked me for being "a true friend and confidant."

Part of me is mad that she won't talk to me about what's going on and doesn't want to waste any more time on this, but the other part of me feels like I've been friends with her for 10+ years and that I should keep trying. What should I do here?

tl;dr: My friend from high school suddenly doesn't want to be friends with me anymore with no explanation. Do I keep trying to save the friendship, or do I move on?