you are angry because you would never do this to someone else. This says alot about your good character. Unfortunately there are some people who will abuse people like you with a kind, trusting nature. Something similar happened to me and like you I was furious and hurt. In hindsight now I realize I was wrong to take it so personally and just realized the parent I was dealing with was not as nice and ethical and honest as I had thought. Over time you will develope a thick skin but you will still never not be surprised that this will occur again because it is not in your nature to do this. Take a deep breath and try not to let it bother you any longer.