You vaguely recall reading something like this in a story. Perhaps it was on the Internet? You try to remember what the person in the story did, but you can't. Eventually, you try a search, and miraculously, you find the story.
In one ending the hero of the story lost his wishing stone. Well, you don't want to do that. In another continuation, he wished his clothes would fit properly, just like you did. Then he was late for school and he wished he wouldn't miss his first period, forgetting what 'period' means when you have a vagina. Since he couldn't reverse his wish, he wished that for a week having the lower half of a girl wouldn't bother him.
His magic stone made everyone think the wish was normal. Yours doesn't automatically, but there's no reason you can't wish for that anyway. So you take a deep breath and say, "I wish I had always been a chick from the waist down, but I remember the change and it doesn't bother me."
Once you make the wish, you stop panicking. You realize you forgot a time limit, but now that doesn't bother you.
You wonder what it is like now that you've always had the lower half of a girl. Soon you'll find out.