It sounds that she does not like this in the first place so you may never be able to make her practice what you want. There are two types of women one who love to dominate the bedroom and others have submissive nature and it seems that your girlfriend is of the second type.
However, whenever you get a chance, talk to her but do it in a healthy manner and just keep letting her know it is your wish that you want her to fulfill it for you. Give her some materials to read about female domination and ask her to see few porn videos of that category so she may start showing interest in it but if the end of the day she does not want to do it then you should drop the idea of convincing her for the femdom.
