How can I protect myself from moral degradation during prolonged contact with destructive people?

In your 1 December 2003 discussion of the morality and legality of abandoning a roommate during a diabetic emergency, you recommend that a person extract himself from that situation as soon as possible -- and that a failure to do so might result in developing undesirable character traits, such as becoming callous to other people's suffering. However, what if a person finds himself in situations in which many people are untrustworthy, immoral, and/or unhealthy -- and that fully extracting himself may require some years? I'm worried about ruining my prospects for making an ideal self because I've had to deal with terrible family, dishonest and unjust employers, abusive coworkers, bad landlords, and so on. It seems like it's going to take a long time for me to get out of this situation. That's a lot of stress and negativity to endure in the medium-term. So how can I protect my psyche -- and prevent myself from becoming callous, indifferent, rude, and so forth?