How can I correct/restore a photo with uneven fading due to a mat?

Question

I'm struggling with a restoration/color correction task for an old photo that was framed with a mat:

As you can see, there's a definite difference in the regions that were covered vs. exposed. I've tried making my own mask, using selection transformations to feather/fade the edges to even out the exposure. But I still end up with an obvious "ring" (the matte is oval) that I can't seem to get rid of. How can I remove the obvious oval shape (in Photoshop for preference)?

(For avoidance of doubt: the blurring on the face is not on the original; it is there to protect the privacy of the individual)

Answer 1

In the blouse area, I selected the area under the mat with the lasso tool with feather set to 20 for your posted image size and used levels (the center adjuster) to match the areas.

In the background area, I used the clone tool and matched the vertical gradient. To smooth things out, Gaussian blur was applied with a radius of 50 to the cloned background area.

I sloppily (didn't spend time on the right side of the blouse and background near the blouse) worked on the right half of the image with the following results. The left side of the image will be a bit more challenging to work on. Perhaps there are better methods, but this took about 10 minutes.