Scarring Near Mouth from Retin-A Use

I had mild acne in the past and the doctor prescribed retin-a 0.5 but she didn't stress how you really can't put it near your mouth. I used it near my mouth and I noticed it began to make a dark strip, like eyeliner smudge. I talked to my dermatologist and she told me it goes away with time and not to do anything to it. It's been 1.5 years and it's still there, it darkens every year in the summer and believe me, I reapply my sunscreen all the time. Is there anything I can do?

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