Someone told me that once youre converted you can go back. But from what I know, one of the reasons why conversion is discouraged is because once converted you cant go back.

A convert cannot leave his Judaism. But the reason we discourage conversion is because Judaism believes in tolerance of other religions, and as opposed to Islam and Christianity, we don't dream of converting people from other religions (and we would appreciate a similar courtesy from them). Also, we want to be 100% sure that the convert is sincere and dedicated to total observance, so we first discourage them, but if they do convert, we accept them with open arms, and even have special commandments to love them.