AITA for confronting my best friend after finding out she’s been hiding a huge secret from me?

So, this is a bit of a complicated situation, and I’m not really sure if I’m being too harsh or if I’m justified in my feelings.

I (26F) have been best friends with Jessica (27F) for about 10 years now. We met in high school, and we’ve been through a lot together—breakups, family issues, job struggles, everything. She’s honestly like the sister I never had, and I thought our friendship was built on complete honesty.

Here’s the problem: Recently, I found out that Jessica has been hiding something huge from me. For months, she’s been in a serious relationship with someone I didn’t even know existed. And it’s not like she mentioned him once or twice—she actively avoided mentioning him at all. She would always go on about her “busy life,” but whenever I asked about her personal life, she just gave vague responses and changed the subject.

I was curious, but I didn’t push her, figuring she would eventually tell me when she was ready. But then, the other day, I ran into her “boyfriend” in the grocery store. I didn’t recognize him at first, but when I saw the way they were acting—clearly comfortable and close—I realized who he was. He was the same guy she’d mentioned in passing years ago, but it wasn’t until then that I put everything together.

When I confronted her about it, her reaction wasn’t what I expected. She seemed defensive and told me I had no right to ask about her personal life. She admitted to dating him but said it was “none of my business.” She claimed she didn’t tell me because I might be “judgmental” and that her relationship was “private.” But here’s where I’m torn:

We’ve been through so much together, and I thought we shared everything. So why hide something this big? She’s always been an open book with me, and I thought we had that trust. But she’s been lying to me for months—sneaking around, keeping him hidden from me—and now I’m feeling like I don’t even know her anymore.

I’ve always been there for Jessica. She’s known everything about me—my family issues, my relationship struggles, my mental health battles—but she hasn’t been open with me at all. She says it’s just her way of “protecting me from drama,” but I can’t shake the feeling that I’ve been shut out of a big part of her life.

I told her how hurt I was that she’d kept this secret, and she said I’m overreacting and that I should “mind my own business.” I’m now wondering if I’m wrong for feeling betrayed or if I’m just being overly sensitive.

So, AITA for confronting her and asking why she kept this from me? Or was I out of line?