Nobody knows what the speaker is, and nobody cares. Sounds like most people have given up and left him for good, even though he's still alive and kicking (he says "I am" twice in this one line alone!).
My friends forsake me like a memory lost;— (2)
The speaker's friends have abandoned him "like a memory lost." His friends are mean; they don't even see the speaker as a person or friend, just as some forgettable memory. Yikes!
And yet I am, and live—like vapours tost Into the nothingness of scorn and noise,—(6-7)
The word "tost" is interesting. The speaker isn't just being ignored; he's been kicked aside and abandoned to a world of "nothingness."
Even the dearest, that I love the best Are strange—nay, rather stranger than the rest. (11-12)
There are lots of ways to read these lines. If the speaker's closest friends are really strange, it could mean they have changed. They have abandoned their old selves as well as the speaker.