I'm the same way, I do usually keep to myself more, and even now I'm more cautious with some friends. I find that a lot of people just don't want to deal with any problems that don't involve them. I found this out recently with one friend of mine as she said nothing about a bad situation that I told her about.

I also need to be sure of someone before I open up to them as I have been burned way to many times by people who would shun me, or worse, stab me in the back by telling others what I told them in private. So now I have trust issues.

It does seem as if most extroverts have no problem telling other people their life story and giving out all their personal info out right away which baffles me as that's just not a safe thing to do IMHO.