Mona,

i don't think there is a such a things as too trusting and i think it is a quality you should endeavour to keep. What you need to change is your boundaries, being clear and what behaviour you will or will not put up with. It's interesting that the jealous girlfriend was actually the one cheating, as is usually the case the person who is least trustworthy is the one most suspicious of others intentions- projection much?

It is a good thing to trust people. Give people your trust, but recognise a Bshitter when you see one (which i know you can) and don't wait around and give them your trust to hit you in the head with over and over again. Because really, you are allowing that and then letting it reinforce the story you tell yourself that everyone is untrustworthy and you need to close yourself off.
It's not that every human is out to get you, or untrustworthy, it's more that you are choosing to be around the ones bad for you. Humans are flawed and even the good ones might inadvertantly hurt your feeling, so it's about building up resilience to that, accepting that people make mistakes.

So now i'm going to go try take my own advice!