It's my super big problem ever since.. I want to wear swimsuit but I cannot because of my armpits.. I hate it! HELP ME... :confused:

If you think your underarms are dark, try using a skin lightening cream on those areas to help fade the discoloration.

Try changing your deodorant. An allergic reaction can cause darkening of skin; seems highly scented deodorants and anti-perspirants are the worst at causing this.
We've had success with using unscented Dove.
Also, be very aware of how your skin feels when wash your armpits, use shaving cream or apply deodorant. If you feel any itchiness, change to another brand.:) :D