<human>: Can you condense the text into a few key points?

Longtime lurker, first time posting. I'll try to keep this as short as possible. 

I am 3/4 Persian and 1/4 Russian. I moved to Europe when I was 12. 
Once I moved, people told me I looked asian and me being the idiot I was, I told everyone I was 1/2 Persian and 1/4 Japanese and Russian respectively. 
This stuck with me ever since and I've always been scared to come clean. For the record, I am in no way obsessed with Japan.

The friends that I have are people online that I've met through a mutual friend. 
I planned on letting them know I was only Persian and Russian, but our mutual friend introduced me as part Japanese.

This is where the tricky bit comes in. I met my SO in a skype call with our mutual friends. 
They told him I was part Japanese days before I had even met him. 
My SO and I love each other to death. We've shared intimate secrets and shed tears with each other. 
We are in a long distance relationship and have met before. 

He told me that he told his mother about our relationship and that she said she likes japanese girls.
I will most likely be meeting his mother for christmas/new years and he will be visiting me in November if all goes well.

I know I MUST tell him the truth and that it's horrible I've hidden this from him, but I just don't know how to bring it up.
 I thought it would be best to tell him in person the next time we see each other, but I can't really think of any ways to approach this. I could be just over thinking it and making it seem like a big deal when it could mean nothing to him.

Any advice would be much appreciated and I'll be more than willing to answer any questions.
<bot>: Lied about my ethnicity when I was 12, lead to my boyfriend thinking I'm 1/4 Japanese, need to tell him I'm not.