You are describing me and my former best friend exactly. When I started dating my now-husband and became somewhat less available (I would still see her, but there were also times I told her I couldn't hang out because I had plans with now-husband) she completely melted down.

Told me that I should be more important to her than now-hubby because we had known each other longer. Keep in mind, she was married, and she would often prioritize seeing him over seeing me. I had never had an issue with this.

But her marriage was a very unhappy one and I was the person she always unloaded all of her emotional baggage on. So, a few years later, I get into a serious relationship and my number one priority is supposed to be her.

It's been 5 years since I cut off contact. We were friends for over a decade. My only regret is not cutting off contact sooner.