When Not to Trust

I have been told by numerous people in my life, but most frequently by my mother, that I am too trusting and too forgiving. On one occasion, a friend forgot to show up for my birthday dinner. I was upset, but I forgave the friend. To this day, my mom still holds a grudge against that person. I used to ignore her because I thought that there is no such thing as being TOO trusting or TOO forgiving. And yet, everytime I choose to forgive and forget, I end up being let down repeatedly by that person. I know forgiving is a good thing, as is trusting, and I still deny that it's possible to be TOO forgiving. But I'm starting to wonder about the TOO trusting. If someone tells me that they are going to do something, than I usually take that person at his/her word. But more times than not, I end up being disappointed. On the less important end of the spectrum, I don't go see a movie that I really want to see because person a says she wants to go see it with me, and then I end up missing it. Or I don't bring lunch to work because person b says that he's going to bring me something. Those obviously aren't that big of issues. But if I can't trust people to do the small things that they said they would do, how can I possibly trust them on the big things. It's one thing to call and say "hey, I can't make it after all." I'm perfectly fine with that. But to just blow it off altogether, just makes me mad. It makes me not want to be around the people who don't keep their word and never apologize for the broken promise or forgetting the plans. So at what point is it okay to be skeptical of what others tell us? Should we continue to trust them when they've given us no reason to?