I'll try my hand at answering this question, but I hope that others give it a shot as well.
I re-read the poem at the site that you gave, came up with my own interpretation of the Shadow, and then went looking for outside links only to find someone who answers the question better than I could. The link is given below.
Grover Smith writes:
With every effort to make the potential become actual a "Shadow" interferes. This, whatever its private value, has in the poem no clear conceptual reference. It implies Prufrockian inertia incapable of connecting imagination and reality, a defect of kinesis, in part a volitional weakness and in part an external constraint. Deathlike, it hinders even the attempt at prayer through which the speaker might come into the "Kingdom" of...
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