What is trust?

I have trust issues. Apparently so did my mother because she always told me, "NEVER trust a man who says 'trust me'" While this somehow holds to be a true statement in my life, I can't through go through life not trusting people. I would be a miserable person. I also know that I have had 3 failed relationships due to trust issues.
What happens when you finally do let down the walls you have built? You let someone "in". How do you define trust? Is it something that you discuss at the beginning of a relationship? Do you sit down and say "these are the ground rules, if you break one, you lose my trust?"

What happens when the trust is broken? Do you give up and chalk it up to yet another life experience gone wrong? Or do you try your best to learn how to trust that person again? How can you trust a person when you can never find out if they are hurting you except by accident? How can you give a person who has hurt you blind trust? Can it be done? If so, how?

A lot has happened in the last 24 hours. As usual, all I have are questions and no answers. Right now, I don't know my ass from a hole in the ground. A lot of hurdles have been thrown in my path. I don't know how to "fix" what is broken. How can you fix something that the other person doesn't believe is broken?

I know this is a confusing post. I also know I may be "airing my dirty laundry" It is a chance I am willing to take to get it help/advise/answers.
I hurt and for once in my life, I don't have anyone to turn to and say "what do I do now?" I don't know how to start healing and stop the pain.