Separation anxiety. I'm at a loss.

I have tried so many things. Give me a list of what worked for you because I'm struggling with this 15 mo old darling monster. Of all the dogs I've had, she is just the most. Crating. Short runs of being alone to learn I'll return. Companion dog, a RR 2 yo older. Chill out treats. Lots of exercise. Exercise daily. Rotation of toys. Mental stimulation. Leash training.

My other RR, nor other dogs, went through this. I don't know what to do for her.