What is someone who for non-malicious reasons always interprets words in the least generous way?

I recently made the unfortunate mistake of apologizing for having been "disingenuous" towards a friend. Luckily my friend was generous enough to assume that I had mis-spoken and ask for clarification; because somehow I had gotten the notion that disingenuous was based on the root of generous rather than genuine. While I had been sincere enough in everything I'd said, it was a lack of generosity in repeatedly assuming the worst intentions in everything they did that I needed to be apologizing for.

Since then, I've been wondering if there even is an English word that means what I wanted to say. I've come up with many individual words that apply to one degree or another, but not something that easily sums up the demeanor I was trying to describe.

Is there a single word that would carry the connotation of "always thinking/assuming the worst" in someone, not in the sense of being deliberately malicious but merely in the sense of being distrustful and bitter and hence reading the worst possible connotations into every situation?