She tests me more than anyone, and it's exhausting

I recently developed a friendship with a girl in my friendship group at school. We never talked much previously but we're very close now. We're both 18 and still in school. She's honestly one of the loveliest people I've ever met, and I trust her completely, but she doesn't seem to feel the same way.

I suffer from depression though I never came to terms with it until I started talking to her. She's the type that is eager to help someone so she welcomed me with open arms. I was shocked to find out she suffered depression too, as well as social anxiety. Though she happily listened as I told her my problems.

A few weeks ago, we were playing cards against humanity online with a group of friends and she kept joking about suicide and self harming, leading me to become worried, but she just smiled laughed and avoided my questions. I kept asking her things, like does she think about suicide, or why doesn't she sleep at night, but she refused to answer them and would purposely confuse me. It was (and still is) very frustrating and exhausting.

She doesn't believe anything nice that I say to her, and she doesn't believe I care, and she continues the same cycle of confusing the **** out of me to the point where I want to give up on her and walk away. With every question I pose, she response with "idk" or "let's not talk about me". It's also impossible to tell whether she's being serious or joking around.

We got into an argument over my skype mood message a few days after our first deep conversation. I wanted there to be no animosity between us and for both of us to be real, but she just flat out refused. I decided that I was done and left it alone, but she sent me a long apology message saying that she was purposely trying to confuse the **** out of me so I'd leave her alone. I felt like such a bad person for wanting to give up on her because of that, but I vowed not to.

A few days ago, we met up to buy a friend a birthday present for a party, and she was doing the same routine all over again. She avoided my questions, purposely confused me, and it generally made me so upset that she was pushing me away. We sorted it out but I knew nothing would change.

We called the same night and spent almost four hours on the phone talking about me and what was up with me, but every time I'd ask her a question, she'd avoid it. Before I went to bed, I wanted to tell her how much I cared about her, but she cut me off and hung up. She was being toxic about it so she could use her purposeful confusion as a means of escaping from my questions.

Everything was brushed under the rug, as usual, but my question is, why does she act like that? What's causing her to be so confusing? And what can I do to make her stop?

I really appreciate this.