This sounds like something similar to what I get to look forward to if/when I tell my mom. My mom lives with us though. My parents were married for about 18 years as well and my dad got back together with his ex-fiance behind my mom's back. Although he isn't poly I am curious as to how my mom, who normally is very open to alternative lifestyles, will react to find out her daughter is interested in men other than her husband.
I have a feeling that a lot of what your mom is feeling, isn't necessarily about you, especially with your dad coming out as poly. Your mom may still equate, if not logically then emotionally, that poly=cheating. With luck it will just take some time and some "proof" of how it works for you.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.