Another thing you can try;
On a copy....
Click on a selection tool and see that there is no feather selected. You're finished with that tool, can pick up the innocuous magnifier for safety.
Ctrl/click on MASK, one layer at a time. The selection line will not be truly accurate as it will not select pixels less than 50% density. Select>modify>expand by some number of pixels (the number to produce a new line which will be the limits of visible image including new feathered edges) to increase what will be the area of image showing through. With white showing as your 'top' color at bottom of toolbox, choose the paintbucket from toolbox and click within the modified selection border. Try one of these;
* Select>feather (by a significant amount)
* Hit Delete key Or--
* With new, expanded selection line active, on mask go Filter>Gaussian Blur, guage amount by preview window.
* This will only effect the edges of the image
I have made a collage by;
Have a layer with a mask, all white. On workarea, make unfeathered selection line around area I want to keep (a generous amount as this will include the feathered amount). Select>Inverse. Click on mask thumbnail, have black be top color in toolbox, fill with paintbucket. Select>Inverse,Select>feather,Select>Inverse, hit delete key.