How do I stop condensation in bathroom?

You have things working against one another. The ceiling vent fan is made to take moisture out, you are putting more in by leaving the tub full of water. The ceiling fan will also pull heated air out, so the warm air you are putting in with the heater is being pulled out the ceiling fan. Empty the tub, vent the room for just a few minutes, then turn on the heater which is a dry heat. That should take care of your problem. In the winter you want a little moisture in the house as heating the air, dries it out. Making it uncomfortable.