Trust and Doubt

As the saying goes: "once bitten twice shy", and I am tempted to add on to that with "twice bitten, forever shy". How can I trust someone who bit me twice without the slightest thought going to a what-if-theres-a-third-coming-up? Doubt kills life. I am not sure whether what I see is truth or my fear manifesting itself. And sour dreams dont help. My mind has the very uncomfortable habit of dreaming my wildest fears, which somehow, don't seem all that wild and impossible when I wake up. The eternal conflict of trust vs doubt.. in the end probably none will win...