It might be a skin fungus (tinea versicolor) that you have already successfully treated, only it takes several months for the white color to even out. If that's the case, you'll just have to wait, because there's no special treatment to speed the process. To tell if it is the case, you will need to see a skin doctor in person. At that visit you can discuss ways to treat any recurrence right away, so you won't have to deal with long-term white spots. But they aren't permanent.
Take care.
Dr. Rockoff
Thank you
Josie