Help with forearm plank

I’m 139 classes in, and for the last few months, plank - especially forearm plank, has felt progressively harder. Yes, I feel it in my core - but it’s my shoulders that are shaking. I feel like my shoulder blades are going to pop off in forearm plank.

I’m frustrated that I feel weaker than I did 6 months ago. I’m also really frustrated that I don’t have enough stability to do a renegade row. That defeats me every time.

What exercises can I be doing outside of PB to strengthen my upper back and shoulders?