Not sure if i should keep seeing a girl or not, She is getting attached too quickly and I only want a fuck buddy and my ex used to be her friend
Make it clear you are not looking for a ship. It's her choice if she want's to invest her time into it.
Shes only 18 though
Abort. I doubt at this point telling her that you just want something casual is going to work. She'll tell you yes while thinking that you're going to fall for her eventually.