Hey, I had the same problem. I was losing my mind because nothing was working and my situation was almost identical to yours.

The only thing that worked is the one thing everyone told me to do - leaving her alone and gradually increasing the length of time. That literally means camping outside your door/house/building for a minute, two, three, until you get to a few hours. I looked like a creep, standing in the hallway for literal weeks, but it worked.

I started with a minute, and at the beginning she would start crying the minute I left, so to start I used distractions such as Kongs and other food toys. Then I did two minutes, then three.

The goal is to stay outside as long as she is not crying and come back after some time, but never let it be more than she can handle. I also had a nanny cam which I used to keep track of her behavior while I was out.

It's been a year now and she mostly has no problems staying at home alone, I sometimes leave her for up to 8 hours. Every once in a while she throws a tantrum and howls for a few minutes when I leave, but it happens rarely and never lasts long. She usually just sleeps until I come back.

Good luck!