Jess I'm a bit like you in that I was always completely trusting - to my detriment. I suppose I was naive. But life has taught me to be more cautious. So I wouldn't say I am suspicious, but I am a lot more cautious than before, when I would simply trust everyone in sight.
One of the experiences that also opened my eyes was when I worked (in SA) in the criminal justice system. After a while I would look at people that I had just met and think "How many previous convictions do you have?"
that was when I knew it was time to look for another job.
Mind you, when I was a juror recently I was again struck by (for example) the mild mannered, well dressed man in the dock who was a prolific car thief (he had 150 previous convictions). If the young me had met him socially I'd have been oblivious even to the thought that he might be anything other than a very nice person.