He told me it was interesting but I needed a more specific thesis, and that my topic sentences were weak (need to use very specific evidence in the paragraphs as well).
Yes, whenever you write fewer than 4 sentences in your intro it is going to seem like the thesis statement is underdeveloped. These first 3 sentences are good, but they touch on many different things. An essay is supposed to be about ONE GRAND THING, one grand idea.
So... ADD A SENTENCE TO THE END OF THE INTRO PARAGRAPH to put all those observations together into one big observation about Stafford's personality, philosophy (as it seems to you) or any interesting observation based on his style. You can even suggest that his style reveals something about his personality. You can't just say, "He uses this device and that device..." You have to add a sentence to the end of that intro paragraph that gives a THEME for your essay, a memorable, creative observation that you make when you really have deeply appreciated this poem (and perhaps compared it to other people's poems to get some perspective.)
About topic sentences and support... try this recipe for paragraphs:
Topic sentence tells the main idea of the paragraph. One paragraph = one idea.
Sentence 2 elaborates or explains what you mean by what you just said in the topic sentence.
Sentence 3 gives an example or a quote.
Sentence 4 evaluates what you just said and comments on it thoughtfully... its implications and what it really means.
:-)