Wow, it certainly is a cause for concern when she is showing anxiety at age 8. Moreover, doc can't properly diagnose a mental condition this young, especially since the brain is still growing (ie: changing). Hopefully with professional help, she may be able to grow out of it.
There is a book about bipolar teens that was recommended to me by the local support group. Not sure if that would help now but good to know what symptoms to look out for when they reach their teenage years.
I bought her this one https://www.amazon.com/Bipolar-Teen-What-Child-Family/dp/1593853181 only $4 on ebay. But it might not apply to you if you're not a teen (I'm 19 so I thought why not haha) but I am sure there are other great ones!