The above poster is correct in saying that the poem "If" is broken into more than one sentence. The first through third stanzas are one complete sentence. It first sentence ends with an exclamation point. Similarly, the final sentence ends in the same way.
As for the interpreted meaning behind, almost, one collective thought, one could justify that Kipling is trying to show the utter confusion that a young man feels when entering manhood.
What seems to be happening is the life of a young man is collapsed into one long string of happenings which all lead to him entering into manhood. Therefore, one could see that it is not one single thing which allows a young man to cross into manhood, but a collection of things that must be understood before he is ready to become a man.
"If" is actually two sentences: the first two stanzas are punctuated with semi-colons, while the third stanza ends with an exclamation point, which is end of a sentence punctuation unless you add a comma after the quote mark that follows the exclamation point.
Some of the answers you seek can be found in a biography of Kipling's life; pay particular attention to the information pertaining to his son John's life.
What do you learn about John? What about John's life is relevant to this poem? What was the pupose of writing it? Was it for John do you think?
The theme of the poem is "carpe diem", or "sieze the day", a common theme in literature which evokes in the reader a feeling that we must make the most of each minute we are given. Does this theme have a tie-in to Kipling's son and the advice he is giving his son.
I know I didn't answer your question, but "IF" (small play on words) you look his bio up online here on e-notes in another window, I'm sure you will have no trouble seeing exacly what you are asking. I have provided you with an additional link to another web site that also has a bio for you.