I also experienced this in childhood with my mother and the "dealing" was similar to others': detachment. Did not understand her attitude at all as a child, just felt it was somehow incompatible with me... only got her (in a complex way) much later as an adult. Even experienced something similar (that led me to detach) with my father, but in his case it wasn't negativity at all, but extreme doses of self-confidence and self-centered attitude, narcissism etc. I did reconnect with him later in my 20's and now we have a very good relationship, more like friends than father-child, but I am still keeping the distance to a certain extent because it can get too much.
Other areas where I sometimes struggle with chronically negative attitudes is work, some of my coworkers that I need to get things done with or even supervise/manage. Really not easy, I've tried all sorts of strategies with such people from being always positive, encouraging, giving many compliments.... to discussing personal problems in a friendly way... to being more formal and diplomatic (never so easy for me).... to tough love and critical approach (this tends to give me the worst results)... to just doing what they are supposed to, myself... Still often unsure, so please if anyone has found good strategies to handle individuals like these in a professional environment while being able to be productive in peace, let me know!