Thank you for the photo.
At this point, after all these years and topical treatments, the best thing to do would be to see a plastic surgeon and have the entire pigmented patch surgically excised.
In experienced hands, given the location of the lesion, this would be a fairly simple operation and would completely solve the issue forever.
Also this would have the added benefit of getting a tissue diagnosis from a pathologist as to what, exactly, the lesion is.
This is what I would do if I were in your shoes.
Does this make sense to you?
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Dr. Rick MD FACS