Using some sort of adaptive contrast adjustment (such as adapthisteq) or thresholding (such as adaptthresh) should help. Using the image in your screenshot (which may well be different from your original image, in future attach the raw image), this seems to work reasonably well:
img = rgb2gray(imcrop(imread('threshold example.PNG'), [40 15 926 180]));
imshow(imbinarize(img, 'adapt', 'Sensitivity', 0.61));
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