How does a person go about trusting another?

Ever since I was young I was insanely cynical about everything, I had a 'rough' childhood, I'll leave it at that, it left me with some major trust issues. It seems like every time I start to really trust someone they betray that trust. I know that I'm probably picking the wrong people to trust but honestly they never give any sign that they can't be trusted until it's too late, actually it's almost as if they gain my trust for the soul purpose of breaking it. I don't understand how people can be like that towards others I try to be good to everyone and I just seem to get stomped on. What should I do? I have considered just becoming a prick but it's just not in my nature to be like that. I need people that I can really rely on that wont cut me down as soon as they get a chance. I have maybe two friends that I can really rely on, every girlfriend I have ever had has treated me like dirt. I put up with just about everything they do to me telling myself that maybe if I gave the relationship time things would get better but they only get worse. I'm not desperate and I'm not clingy, I'm not ugly and I don't think I'm a bad person. I treat them well, I do all that I can for them and just get beaten back down to a cynical pessimistic shell of a person. How can I get out of this rut? I have no large self esteem issues or anything like that but I'm also not an egomaniac. I'm just a normal good person that tries to help others..