I'm extremely new to this site, and haven't actually gotten any of the files to work yet, but I thought this would be an interesting concept.
Basically, the curse would create a second personality inside your head, and this personality would be a normal, everyday girl. She'd start at around ten years old and wouldn't be able to do anything except chat with you in your head. She would even have her own name.
Then, every week she would get a year older until she became the same age as you. And as she gets older, she gets more and more control over you and your body. For example, maybe around the second week (12 year olds), you might find yourself suddenly playing with dolls because your other personality wanted to.
Or you might find yourself automatically sitting when you go to the bathroom. Or maybe feeling an urge to grow out your hair, then put it into pigtails or a ponytail.
And as the weeks went by, the other personality would be able to take full control of your body and push you into the 'back seat'. She'd be able to do this more and more often, until finally she would have total control of your body, while you would be like she was in week 1, just a voice in her head.