Selectively secretive is a good way to describe it in practice. In real life, I'm super tight-lipped and don't tell anyone anything. My parents barely know things about me, and I have no plans to cultivate any friendships with co-workers, so they don't even know anything about me either. If I have no plan on investing myself to you, then you get nothing.
The reason for this is because honestly, most of my true personality has to be a secret. It's not something that many people would understand. I am plagued by an abundance of psychological disorders, and I just can't comfortably express myself without making them miserable or eliciting some bad reaction. It isn't just the disorders though. The good aspects of me are very mental, deeply spiritual, and complicated. I take things super seriously as opposed to the casual demeanor of most that I observe. Also, I'm always mired in some other world, thinking about things that don't exist here, that people find it off-putting when I reveal that. Basically, I have to keep my entire real self a secret.
Consequently, my secrets are mostly confided here, to all of you, and wherever else I am on the internet. I'm forced to by the way my brain works. It constantly has to be spewing out things and expressing or else it all bunches up inside of me. I find the internet to be the best place to do that because people generally don't respond to anything I say, due to it being too long, and they generally don't care either. I don't have many things that would incite anger to say. The things I write, I expect people not to read. If they do though, I don't mind. It's a nice bonus that makes me feel like I'm being listened to.
Of course, I won't post everything. There are some things that I'd rather not talk about even online (due to the stigma associated with those things), but by my true nature, I am an open person. The one person who gets all of me, and every single secret I have to divulge is my fiance Lunaris. Overall, I'm not much of a secret person. If I didn't have to worry about stigma, had more reason to trust people, and if it wasn't healthy to have some kind of emotional boundaries between you and others, then I'd happily divulge everything I could like an open book.
In short though, I'm all or nothing. Super secretive or completely open, depending on the context.
Though I hate keeping secrets myself. You can read my face like a open book, and I find it absolutely painful to lie. Of course, just because I hate it, doesn't mean I won't do it if somebody asks me to. It's just by asking me to keep a secret, you're asking me to have this terrible painful burden inside of me. But you can generally trust me. I won't tell if it's super important, but I will say as much of what I can without actually revealing it. Something along the lines of: "My friend told me to keep this secret, and I can't tell you. I'm sorry." That's if it's somebody who is important that's asking about it though. If it's some random stranger, they won't get anything out of me.
For some reason, people like to tell me a lot of things about themselves out of nowhere. Sometimes deeply personal. Both in real life and the internet. On the internet, I presume it is because they are comforted by my generally sincere nature and find someone so emotionally complex to be an individual who won't judge them. And in real life? Maybe it's because I'm so quiet, they don't think I'll say anything bad to them. Which I won't. So maybe that's it.