How did you find out that she was making fun of you, is this a reliable source? Sometimes people take a negative truth and wrap it in lies to manipulate you. Maybe she wasn’t saying anything with malice, maybe she was.
I hate to say it, but yes, you’re right, you do have to be careful with who you share information if that information would be helpful to you should it be used against you. Once you come to terms with those feelings, no one can use that against you.

I have trouble trusting anyone as well since my ex cheated on me and treated me so badly by making fun of me, calling me names and throwing things at me