I consider the occasional betrayal or other failure of trust to be a small price to pay for the ease of mind and comfort of being highly trusting.
I know that in the course of my life I'll fall off my bike several times and I'll get back up and keep riding. It's part of the cost of riding a bicycle and is easily worth the enjoyment of riding. I don't fear riding. I trust my balance even though it'll fail me from time to time.
Same thing goes with trust and life. It's fun while it lasts and the moment when it fails is just that, a moment. You get back up, adjust your trust a bit, and keep on enjoying the comfort of friendship and the joy of a life with less fear.
What do you REALLY have to lose? What can a failure of trust REALLY take away from you? I mean among things that REALLY matter? Your heart knows, follow your insticts and don't sweat it if you're wrong sometimes.