There are a lot of lines that indicate his nervousness. Take a look at this one:
"there will be time/To prepare a face to meet the faces that you meet."
He is so nervous that he feels he must prepare beforehand for whatever it is that he is going to say. He feels intimidated by the people that he meets at these social gatherings, and thinks, studies and analyzes beforehand how he should act, what he should say, how he should look. Consider the next passage too:
"Time to turn back and descend the stair,
With a bald spot in the middle of my hair—
[They will say: "How his hair is growing thin!"]
My morning coat, my collar mounting firmly to the
My necktie rich and modest, but asserted by a
[They will say: "But how his arms and legs are
He is constantly worried about what other people are going to say to him. Indeed, he is even imagining chickening out, and having everyone stare at him and critically analyze him as he leaves the room. He is afraid of that analysis, afraid of chickening out, and stressing out about it beforehand. Then, he wonders to himself,
"Should I, after tea and cakes and ices,/Have the strength to force the moment to its crisis?"
He is stressed that his question or declaration is too serious, too deep and jarring to ask in the mundane routine social events of tea-taking. He keeps worrying about this throughout the entire poem, calling his declaration a crisis, murder, creation, rolling the universe into a ball; he is exaggerating the impact of his declaration as a way to get out of saying it. When people are nervous, they do this-exaggerate and blow things out of proportion. He then calls himself "foolish", "obtuse", and no great speaker or orator like "prince Hamlet". He is so insecure about himself that he is insulting and skeptical, worrying that a "fool" such as himself might be rejected. Those are just a few lines that indicate his nervousness; I hope that helps!