(Final edited image: Parrot Tulips, R. Garriott, oil, 24"x24")
In the top toolbar, choose Image/Image Adjustment/Brightness-contrast. To darken, move the brightness pointer to the left. In this case I've reduced the brightness by -14. This allows the background to blend in with the dark areas at bottom and left with no noticeable line.
Release the selected area, and you'll have your result. If you get a 'line' or the fix doesn't blend in smoothly, undo (Ctrl+Z), try again, and adjust your increments.
If the glare covers a large portion and is noticeable over areas of light and dark, I would suggest rephotographing as a first step.