I don't trust people

very easily. Perhaps it is because I have been let down too many times in my life.
When I say I will do something for someone else then I will make every effort to actually do it. Yes, there have been times when I have failed - but I hope there has never  been an occasion on which I have said, "Yes, I'll do that" and then not thought about it again, not even tried.
I also try to support people. I am a firm believer that, if you speak ill of other people, they will speak ill of you. Even if people let me down I try not to speak ill of them. What's the point? If they are already feeling guilty about letting me down they are just going to feel worse. If they don't care then they are just going to think the worse of me.
I know not everyone else feels that way. They don't notice. They don't seem to care. It simply doesn't bother them. They assume that their relationships will simply continue as before.
When I was growing up there were people you were told you could trust - religious leaders, teachers, the police and the medical profession were all considered trustworthy. You went to them for help. I wonder who we trust now? Who do children trust? 
Several years ago my late friend E... asked me to take on guardianship and power of attorney duties for her. I asked, as I should  have done, "What about your sister?" The answer was "No, I trust you."
At that moment my friend placed a burden on me. If I had told her "No, I can't do that for you" her trust in me would have been misplaced. In the end the duties were not onerous. E.... did not live a long life. 
Recently I have had to face another problem - over someone who has been causing problems for some time. I still wonder if I did the right thing but I felt a point had come in the (non) relationship where I had to say, "Enough is enough."
I told that person "if you turn up at the front door and say you are sorry I will still invite you in and make you a cup of tea but, until you are willing to do that, please leave me alone."
Unreasonable? I hope not. She has let me down badly. She has let other people down too. I no longer trust her and I worry about the harm she has done and could still do. I know other people feel the same way.
I also know she will no longer trust me, probably has not trusted me for some time. No, I didn't let her down but she chose to believe I had. Until she sees that the relationship cannot be repaired - and I cannot trust her either.
Trust is hard to gain and all too easily broken.