And the silent isle imbowers The Lady of Shalott. (lines 17-18)
We think this line really rides on that big vocab word: "imbowers." This means to close up in a bower, which is an old term that refers to a lady's private room. In another kind of poem, this could be associated with protection, keeping someone safe from the outside world. But in this poem, we think it's a lot darker than that. This bower is really a prison, even though we don't know who put it there. Even that bit about the "silent isle" emphasizes how lonely and isolated and cut off from the world the Lady is.
She hath no loyal knight and true, The Lady of Shalott. (lines 62-63)
She's not just isolated physically, but emotionally too. She wants what anyone wants, companionship, comfort, and love. She can see these things in her mirror, she can watch lovers stroll by, but she is cursed to be alone. That's what this line is all about. A knight, in this world, would protect and serve a lady, like the red-cross knight on Lancelot's shield (lines 78-9). In a sense the poem suggests that knights and ladies belong together, and a lady on her own is incomplete. You might have some problems with that idea, but that seems to be the message the Lady is getting.
"I am half sick of shadows," said The Lady of Shalott. (lines 71-72)
Here's the clearest expression of frustration from the Lady. In this moment we learn not just that she feels isolated in her shadow world, but also that the isolation really hurts. She wants not only to watch other people but also to join them.