What should I do? She's stealling my bff?

You said you and your friend aren't that close any more so if i were you i would started hanging out with them more. Also, if you know what he/she likes to do (like surf,golf,etc...) then do that with them. You may not like it but they might see it as something in common and decide that it's something that they would like to do with you again. So you and your best friend grow closer.