Taking classes, if it's something reasonable for you to afford, would be a nice start. You could also look up video tutorials online for simple things that don't even have to be clothes.
Here's a page of beginners sewing projects you may be interested in going through and perhaps make something special for your mother as well.
Scented fluff pockets! Super simple to make, fill it with Potpouri leafs and make your room smell awesome.
You can put some time into learning simple crafts, keep yourself occupied,perhaps impress mother dearest with your budding talents, and when the time comes you are old enough to handle convention business on your own you'll have craft experience behind you and your mother will know you respected her wishes in you not participating in something she views as dangerous.
You may also consider this, gather a group of friends up who would like to go to the convention as well and see if one of their parents would be willing to chaperon for a day. You still may not be allowed to cosplay, but if there's a group of you with an adult around your mother may feel more comfortable with this.
But you cannot push the issue. Being whiny, bratty, or otherwise throwing tantrums will not improve your situation. Your mother isn't suddenly going to have a change of heart if you cry and whine. If you are going to be unable to go to a convention, that's the end of the matter and you need to try and accept that responsibly.
Scented fluff pockets? That sounds awesome.
Keep in mind that you can cosplay at places besides conventions. Since she's really opposed to conventions, maybe arrange a cosplay picnic that she can come to? She'd probably feel much more comfortable there since it's more "normal."
She would never.
I do suggest getting involved with your school's theater department or community theater. That way you can assist with costumes in a school/community sanctioned enviornment that also looks good on a college application.
Another idea is to join or start a Anime/cosplay/World cultures club at your school/library/community center. It shows commitment and responsibility to go through the proper channels to start a club, then plan a trip to a nearby con not to mention fundraising for the trip.
I probably couldn't do any of those either. Besides, we aren't allowed to make our own clubs at school.
It looks like I am going to have to wait--or at least until I'm about 15 or 16...
*sigh* Thanks for all the help. I'll send a message to get this account deactivated pretty soon. ;w;