I get the impression that you don't have a fan that vents the moisture
out of the room. If this is the case, your best bet is to use exterior
grade paint that will be more resistant to humid conditions. Strip and
sand what you have now. Then prime with an oilbase and finish with a
coat or two of exterior latex. It will be smelly, so keep the
window(s) open as much as possible. Good Luck!