1. Being a spriter will not require you to move to Japan. Did you know that there are some video games made OUTSIDE of Japan? To be honest, the industry is booming moreso in the US than it is over there, so... you know... I think you're already in the right place.
2. If you don't go to college/university and get a degree in art, you'll never be hired as a game artist unless you have one hell of a portfolio which... you probably don't if you aren't heading onto post-secondary education for art. Just working in MS Paint on your own for 8 years isn't enough. Even if you're only working in something specific like spriting, you'll likely be required to have a very wide background in art to get hired by a game developer as a spriter.
That said, your mom likely doesn't think you're responsible enough to move out on your own and considering you don't even seem to have made much research on your career choice, I'm inclined to agree with her. But if you're adamant...
Do your research. Figure out exactly where you're going, what you're doing when you get there, and what your long term and short term goals are. Look into the cost of living in the area, figure out what income you and your bf will have if you live together, and figure out how you're going to obtain the funds for that. (Living off disability, if you don't need to, is horrible btw--get a job if you can, seriously. :|) If possible, sit down with your family AND your boyfriend and discuss options, what's going to happen, and what your plan is.
Your mom is probably just worried about you and doesn't want you to run off and get in over your head. Leaving home is a BIG DEAL and if you aren't ready for it and don't take it seriously, you can rack up some serious debt. But if you learn the facts and go out of your way to make sure everything will go smoothly--and communicate all of this to your mom, she'll probably be more comfortable with it. You are old enough to make these decisions for yourself but it doesn't really sound like you're mature enough and that's the big thing here.
Personal question, I know, but what are *you* collecting disability for? Why can you not go get a job now but if your boyfriend weren't an issue, it'd be okay to pack up and leave for Japan to do spriting? :/ That sounds a hell of a lot more stressful and debilitating than, say, working retail for a bit.