How can I honestly answer questions about these professors without coming across as bitter (which I admittedly still am)?
By explaining what happened to you, and perhaps to other people, and sticking to the facts - rather than providing your conclusion about those Professors' character. (And note that the way events made you feel is also a fact.) Let the person asking you reach their own conclusions.
Also, by qualifying your answer with a mention of other people who have had better experience with these Professors, whom the person asking you the question might also consult.
However ... be advised that not embellishing and not being ultra-positive will in many casesoften be perceived as disloyal bad-mouthing, by manycertain people. Honesty has its price and no good deed goes unpunished - in many institutions.