I think everyone has given you some great advice on how to handle your "switchiness" when you are in a relationship with a sub. But perhaps this girl in particular just isn't ready to have a D/s relationship with you.

If that is the case, trying to become more Dom-ish with her - setting rules, etc. - is probably more likely to scare her off entirely than anything else.

If you and her are happy being friends, then be friends.

Stop being concerned about topping her all the time, and just be happy with what you have together at the moment.

Just my two cents.

lily