FSP usually has more spasticity rather than weakness by history and exam. It may be worth looking at your spine with an MRI to amke sure nothing is going on back there which may need surgical intervention as you say it's getting worse. Many other things can cause spinal cord problems such as B12 deficiency and could be evaluated with lab studies. Also, it could also be primary muscle problems such as a myopathy that could be detected with EMG. Consider seeing a neurologist to get a formal exam which should helpyou get started int he right direction.
With your family history of spastic paraplegia, the diagnosis is certainly a consideration with your symptoms. There are various modes of inheritance with different types of FSP-not just the type in which women are carriers. In addition to that type of inheritance pattern (which is very rare), regardless of gender, an affected parent has a 25-50% chance of having a child with disease depending on the type of inheritance. You're right about the spastin gene test, it only accounts for 50% of affected patients. Unfortunately, there is no other definitive test, only a clinical history and exam, and no cure at this time. If spasticty is a big problem, a baclofen pump may be helpful and other supportive measures can be taken. But no cure. Good luck.