Why can't I just be nice?

I had a fight with my boyfriend last night and he told me that I'm too critical, uptight and angry. He's right! I have no patience for rude, dishonest and selfish people. I'm a quick judge of character, I either like someone or I don't. He is friends with everyone and cares about even the lowest sbags of the earth. When ever I think about being nice to some of the people that he is, it makes me want to scratch the ds! I get flustered and I get this terrible sick feeling in my stomache. I feel like if I was nice to these people then I would hate myself for being such a dishonest person. On the other hand, I know he's right that I should care more about other people but how do I fake it and still feel good about myself??