If I were you I’d embrace what I’ve got and get a nice long tan using moisturizer so it doesn’t burn. That way you can even it out
I would imagine that you could prevent further damage and darkening of the area by applying sunscreen to the area- this may not avoid it all together, but it should help. It's probably a good idea to place sunscreen on any exposed skin, given that you have damage in one area already. It's also ok to go to another doctor to get a second opinion if you are concerned about the accuracy of the first diagnosis.