Why might a friend suddenly act distant and withdraw?

For context, I’m 30(F) and she’s 36. I thought we were past this and we were mature adults. This feels very high-school.

So l just feel really hurt because someone that I considered a good friend has been distant lately. She had been going through a bad divorce for a few years and I was always very supportive and kind towards her. We never had any drama, I was always kind and loving towards her, and we've never had an argument or issue. I always thought we had an extremely healthy friendship, that's why this hurts so much and I feel like someone punched me in the stomach. I don't feel like I did anything wrong. I can't think of a single thing. I would never even tell her about any of my problems because I didn't want to bother her with any of it. I was always the friend with the listening ear.

She would even refer to me as her "bestfriend". I tried to keep the friendship alive, but I noticed that she was always too busy for me, but I just assumed that she had a lot going on and she does work a lot. Long story short, she's become more distant and I found out today that she had restricted me from her Instagram stories this whole time. I checked her stories through my boyfriend's account and she was posting pictures of her other friends and going out to a restaurant and the mall.

I feel like saying something, but I'm not. I'm just going to consider the friendship over and dead. It's just really hard because I really don't have a lot of friends. I honestly have had trouble keeping friends long-term in the past and it's been hard making new ones. This was my longest friendship, and we had been friends for eight years. It's hurtful. I hope new friends are in my future but I feel pathetic having none. Why does this still happen with women even when we’re well into adulthood?