Hey Sarah 22!
I understand your situation quite well! My Mum is like your Mum. "Why does he have to be Japanese? I don't wanna have half-Japanese grandchildren!" Sounds quite familiar, doesn't it? It is a really hard situation, for you even more than for me. I am already studying in another city and my boyfriend is living in Japan, so my family doesn't have so much influence on me anymore. But you are still young, so your mum might be thinking your relationship is like 'kid's first love play'. Even my mum is thinking that and I am 21!
It must be very hard for you, but if you really love that guy, you should decide for yourself! You seem mature enough to me to handle that situation quite well.
Wish you good luck!