How much pain does D feel from years of discrimination as a dhampir? It's definitely implied he's lonely but it's hard to tell if he is resentful about it at all, since he almost always has that cold stoic front. He might be partly 'it is what it is' by this point about it. (Also, hope I have not been annoying you with all my messages XD)
You have not been annoying at all!!
I’m inclined to agree with the notion that D is on the ‘it is what it is’ side of things. Like, it absolutely sucks, but there isn’t much he can do about it so why let it bother him personally when he has more important things to deal with?
Granny Viper gives us an idea of what it might feel like - but again, D is D. We have an idea, but we’ll never truly know until such time when Kikuchi-san gives us that side of D in another story.
What I can say about his feelings toward being discriminated against, on a person-to-person aspect versus society in general, is if someone is being openly shitty about what he is…he will make his feelings known one way or another. As time has progressed in the series, D seems to have become more comfortable about what he is, on top of already being one to not take shit from others.
One case in particular - Lilia from White Devil Mountain…an absolute asshole who had no qualms about pointing out the fact the D’s a dhampir, and rubbed it in his face by basically shaming him in front of other people. At one point in the story, D basically whipped out his sword, stopping just short of slicing into her face, and gave her a look that said ‘shut the fuck up’. Of course, being the asshole that she was, Lilia’s like ‘wtf was that for?’ Another character then had to point out that D had left a cut on her nose and explained to her ‘you pissed him off, that was a warning…you got off light…next time you might not be so lucky’.
Obviously D doesn’t like it, however, he has his ways of tuning it out…but he’ll straight up cut a bitch if he has to.