Sentence:  I see by your eagerness and the wonder and hope which your eyes express, my friend, that you expect to be informed of the secret with which I am acquainted; that cannot be; listen patiently until the end of my story, and you will easily perceive why I am reserved upon that subject. I will not lead you on, unguarded and ardent as I then was, to your destruction and <WSD>infallible</WSD> misery. 
Meaning of the word marked with <WSD> tags:  Certain to produce the intended effect, sure.