I'm really glad to see that you get to like poetry more and more. It takes time and efforts, but I believe you will find the efforts rewarding.
You said here about "Harlem" that "As far as I am concerned, I think the speaker was very confused about the real figure of the dream itself. However, he used a lot of concrete object to compare to it, but he didn't find the
answer successfully." This is a point I haven't thought about before--what if the speaker is, like you said, just confused? I then re-read the poem and the shortness of the lines--for me-- shows a kind of certainty. From another perspective, then, I would say that the questions are intentionally made to stimulate our thinking and to challenge to illusions of American Dream. The questions, in other words, are powerful critique for me, but not a sign of the speaker's lack of answers.