I agree that it is about control/change. If the kilts were part of your life before you met her, they are relevant. It is something which you need to work out.
Look at it this way... The kilts are not a destructive hobby or exspense. If you were gambling, abusing drugs, dropping whole paychecks on strippers...
Explain about the clothing, but also the community and the benefits you (and she?) get from your having a kilted life as an outlet. There are far worse hobbies & activities that you could be engaging in.
Good luck!