i told my friend something about someone else and i literally said don't tell her so many times and she said she won't but I'm worried?

All you can do is hope. You've already told her countless times you want this to stay between you and her. The more you badger her about it, the greater chance she'll be irritated enough to tell somebody else. So, STOP. Leave that topic alone and in the past.

Next time, quit gossiping about people or telling their business to somebody else.