Discretion. Where's the line?

I struggle with discretion.
This may be due to my former job as a journalist, or as the newsdesk used to jokingly call ourselves, a professional gossip. And even now, working as a PR my entire life pretty much revolves around giving people information that I would like them to broadcast to the nation.
So it seems, I just don't know where to stop. I'm very open with colleagues if I'm dating someone, I admit I can be very open with my friends about my sexual preferences, I'm just a very open, honest person, I'll tell anyone anything if they want to know, give me alcohol and I'll tell people anything even if they don't want to know.
And on some levels, I think it may work for me. People often confide in me because I have been so open with them, it signals trust I guess doesn't it? I find it very easy to make friends because I am so open with who and what I am and how I am feeling.
And I would far rather be friendly and open than closed, snobby and unable to trust.
If someone does tell me something in confidence, I do, on the most part, keep my gob shut. I admit, I may tell my sister... but she is much better at discretion than me and knows when to stop talking.
Similarly I'm careful about bitching. A sorry lesson learnt from years of teenage angst. I have to really trust someone before I am open with my dislike for or upset over a colleague or somebody in a circle of friends and often I will ask them to keep my comments private.  I admit sometimes, if it is a mutual friend or colleague, I should think more about the situation I am putting them in and that I may be making them feel uncomfortable just by knowing this information.
Similarly, I sometimes worry, do my revelations make people feel uncomfortable? When is it alright to tell people about that crazy one night stand or drunken shenanigan? How close a friend do they have to be?
I especially worry about co workers, how much do you give away there? (N.B. I know enough not to tell co workers about crazy one night stands.) Where is the line between appropriate and chatty and over sharing? 
Also, I will never be able to pull off mysterious. I so envy those people who have an aura of mystery about them. Who play their cards close to their chests, who you never quite know what they are thinking. It's sexy and intriguing. Everyone knows what I'm thinking because I happily blurt it out. Not so sexy and intriguing.
It has to be said that social media was the final nail in the coffin for discretion. If a woman can tweet about her miscarriage and Belle Du Jour can blog about her sex life, what chance do I have with keeping my gob shut?
Shockingly, it only very recently occured to me  - while reading about how Kimberly of Girls Aloud has only ever had two lovers - a persons sex life isn't anyone's business but their own! It's meant to be private! I know, such is the loss of discretion in the modern world that it took me until I was 26 to realise that my sex life belonged to me alone, I didn't have to tell the world about it and also, if people asked, I didn't actually have to answer them honestly.  If a new boyfriend asks about my past, I don't technically have to tell him anything that doesn't relate to sexual health and contraception. I can just reply something vague like: 'A couple of long-term boyfriends and a smattering of flings.' Rather than giving him dates, times and scores out of ten. 
As someone who isn't particularly discretion savvy, I have decided to hunt out advice. What is the correct way to behave? How much information should I give out to the world, does my openness make people feel uncomfortable or gossip about me in a way I wouldn't like? It's certainly something I am to be more careful with in future. I just hope this doesn't make me less fun!