Wanting a friend to be vulnerable

I have a close friend who refuses to be vulnerable with anyone. I found her sobbing in her room the other day and when asked what is wrong she continually responded with “I’m fine,” and we haven’t talked about the incident since. I find myself being frustrated with her, because she clearly is not fine, and talking about whatever is bothering her (being vulnerable) will help to alleviate those worries. I know I shouldn’t be facing this issue with frustration, but I don’t know how to change my opinion and what my opinion should be. I also started to think that maybe she thinks I’m a bad person, and that is why she doesn’t want to talk to me. I realize that that is projecting what I think about myself onto what I think her thoughts could be, but I’m having a hard time knowing how to fix it and if these thoughts are actually valid. Lastly, I feel super distant from her because I feel like I’m missing a whole part of her life that she refuses to talk about with me, which is her anxiety/depression.