Oversharing with Someone I Don't Trust

So there's this person that I talk to. She's an acquaintance, but I don't trust her. I keep reminding myself that whenever I have a conversation with her, I shouldn't overshare. But somehow she makes you feel comfortable with her, and I end up sharing way more about my life than I wanted to. I'm tired of making this mistake. How can I seriously improve this? I know I shouldn't associate with people I don't trust, but this person is a coworker, so the association and interaction is inevitable. Sigh.