Trust is an important if not a key to every relationship, whether we talk about friends, family or boyfriends/girlfriends. And I was always the person that wants to see the good in people so I trusted people, maybe too easily.
A few years ago, when I was still in high school, I had a best friend. we used to do almost everything together: we hang out a lot, we talked on the phone all the time and most important , we trusted each other or so I thought. My mom would always argue with me that I spent too much time with her.
My mom has an apartment that she rent sometimes and because in that period it was empty we used to go there and at some point we thought we should make a spare key to come anytime we want because my mom didn't want us to go there that often ( and I know - IT WAS A BAD IDEA, but I was just 15 or 16 years old so I didn't know much at that time).

From here I guess you could see the outcome: my bbf went there without telling me (I let keep the key so that my mom wouldn't find out- it was USELESS in the end). The real problem was that she went there with her boyfriend and my mom found them there.
I remember the day very clearly. I was in at school went her boyfriend came to tell me not give my bff's parents number. At that time I didn't know what happened so I said yes. But when I got home my mom lectured me and grounded me , but most important I lost her trust. And to top it all, the next day my "bff" walked past me like we just strangers. And after that we never talked again. It's been years since we've talked and to be honest I don't want to. In that day, I lost my trust in her and when that happened I became just a teenage girl with big trust issues.
I couldn't open myself the same because I was afraid it will happen the same thing. So I had friends but not too close friends, if you what I mean and it took me a few years to accept what happened and to move on if I can say so.
Now I'm almost 20 and the past still haunts me sometimes. I have few close friends and I am grateful for them and I try everyday to not let my fear overcome me because I don't want to live in the past anymore. I want to live in the present and to prepare for a nice future. Because now I trust in myself more than ever and I won't let anyone take that away from me.
Trust me ;)