this is your first disappointment and most likely it won't be your last but i hope it is.
first take a breath.
there will be more options. from what i have observes, PWS may be slow at that age so your child is not in any danger. you said her health is ok otherwise, that is really important.
you haven't let your daughter down. The doctor could definitely have a better manner and been more specific. Each case may be different that you read about on the internet. Best thing would be to get another opinion if it will help you feel better.
just remember, she IS ok