FH is not high cholesterol that crept up on you. It is a single inherited gene fault, and it has been raising your LDL since childhood.
Your liver clears LDL cholesterol out of your blood using receptors on its surface. In FH those receptors are missing, reduced in number, or do not work properly, so LDL stays in circulation instead of being removed. The result is a cholesterol level that is high from the start and stays high, regardless of how carefully you eat.
That last point matters, because people with FH are often told for years that they simply need to try harder with diet and exercise. Both are worth doing. Neither addresses the underlying fault.