Yes, I open up to people about a great many things. And sometimes, in order to "bare my soul" (yeah, it is that "diarist" thing), I probably reveal too much to the public and occasionally get my feelings hurt.
I simply haven't found the same experience you have. I can count on two hands the number of people in my life who have purposefully tried to exploit me (if that many).
The rest of the time, people have been decent and trustworthy; or else they've simply been human and made some mistakes but were willing to rectify things and at worst retreat to a "neutral" or "live and let live" stance.
So I cannot really explain why you've had a completely different set of experiences. I can't tell you whether you are simply in a different social setting where the people are less trustworthy; or whether your approach/attitude is negatively impacting their reactions/treatment of you; or whether you judge people far too harshly as to what "taking advantage of you" means (or I judge them not harshly enough); or whatever else we can conjecture. There are so many possibilities for this, and probably it is a combination of things.
Still, my locus of control has shifted over the years from outside of me to inside of me. Put in simple terms, people only hurt me if I allow them to. So I can feel free to be honest and open with people, and if someone reacts poorly, well, I do not need to choose to allow fear of that to control my life. It's their loss, not mine. And the rest is just emotional hurt, which most of the time goes away.
I refuse to allow others to keep me in a box and not expose myself and be alive.
EDIT: Anyway, don't let my comments stop you from venting and getting it all out.