How to deal with feeling betrayed?

I recently found out what my roommate (cousin) been saying about me through an old friend that's really close to her now. and I swear I've always felt like we had a super strong bond, I've never said anything bad about her she's like a sister & I thought she felt the same about me but My guess was wrong😟 I feel like talking to someone about it but who? I mean she's the ONLY person I'd tell everything to and trusted with anything.