Good day!
Could you please post a screenshot with the Layers Panel visible?
I created a new brightness/contrast adjustment layer, and pasted in her cut out rump in this layer.
What good was pasting part of the image into that Layer (I assume you mean its Layer Mask) supposed to do?
You could try blurring the Adjustment Layer’s Layer Mask or painting into it with a soft Brush.
Regards,
Pfaffenbichler