Suggestions? Just do your best to be friendly and reasonably accommodating. That's all anyone can or should ask. If you can keep your side of the street clean, then you have nothing to apologize for, feel awkward about, etc. Maybe you're not the most social person on the planet. Who cares? You're doing your best. If others don't see it and appreciate it, that's their problem.
You may be making a classic mistake, which is comparing your insides to others' outsides. Like in your response to the person who didn't return your "hello." You're having a very complex emotional reaction to what ultimately amounts to very little -- too little, actually, which is the problem. This person did something or didn't do something, and you're confused and seemingly deeply troubled by it. But, you have no idea what was going on inside him at the moment. Maybe he hates you, maybe he has a secret crush on you, maybe his pet bird just died ... whatever. All that counts is that you did the right thing. That's what you can control, and you did a fine job. You can't control the response, so you have to let that go. (Well, I guess you don't have to, but you'll live more sanely if you do.)
You sound like a very nice person to live with; I'd be satisfied with that if I were you. Not many people can say that!