What impression do you get about the poet when you read the line "And never breathe a word about your loss"?

To best understand Rudyard Kipling through his poetry, a reader might examine the line "And never breathe a word about your loss," specifically in the context of the rest of the stanza and its application the Victorian period. In short, Kipling examines themes of personal persistence, individualism, and pride when he explains one must accept the defeat of some of their worthwhile risks with stoicism and perseverance.
The entire quatrain that begins the stanza is:

"If you can make one heap of all your winningsAnd risk it on one turn of pitch-and-toss,And lose, and start again at your beginningsAnd never breathe a word about your loss;"

To paraphrase, Kipling examines that to be "a man, my son," the closing words of the poem meaning to be stable and mature, one must be able to take what they have earned, their "heap" of "winnings," and invest them in further greatness without fear, the "risk...on one turn of pitch and toss." While not always successful, people, especially young people who are still maturing, the addressees of the poem, even losers must be proud enough to be free of complaining, or said more casually, whining. This advice leads into the next quatrain of the stanza:

"If you can force your heart and nerve and sinewTo serve your turn long after they are gone, And so hold on when there is nothing in youExcept the Will which says to them: ‘Hold on!’"

Not only does one match their loss with pride and stoicism, hiding their pain, but they respond with perseverance, forcing their body physically, their muscular "sinew," and spiritually, their passionate "heart," toward further success.
This explication examines a few central ideas of the Victorian period, which lasted through the later half of the 19th century, including proper conduct, individualism, and pride. Firstly, the Victorian period is known for its installation of manners and hidden rules of conduct, what we might call "bourgeois" etiquette. The Victorian era is the era of parasols, corsets, and ballooning dresses m,any of us have in mind when we think of stereotypical high society. The criticisms of high society that Wilde criticizes in The Importance of Being Earnest dominate this time period. Thus, when readers look to "never breath[ing] a word about your loss," they gain an insight into the Victorian aristocracy Kipling would have been familiar with that valued being tame, impassioned, and, again, proper.
Secondly, though, the Victorian period is a historical period in which the middle class grows in England, and more opportunities exist given the Empire's booming colonial economy. Thus, "never breath[ing] a word about your loss" calls out the values of rugged individualism, what we might call "pulling ourselves up by our bootstraps," that dominates the time as well. Simply, Kipling suggests that those that are successful take risks no matter the consequences, and that is what makes a man in the period in which he is writing.
Finally, the line evokes a sense of pride, which might specifically be understood as a sense of national, or imperial pride, of the Victorian British Empire. Historically, the Victorian period is a period in which the English expand their empire beyond the West Indies to also include India itself, where Kipling is from, and large swathes of Africa. This development allowed an opportunity for many young people to stake their claim as foreign officers, envoys, or soldiers for their own gain, an example of Rugged individualism, and for their nation, the Crown. Simply, "never breath[ing] a word about your loss" and pressing on with your "heart and sinew" evokes ideas of colonial expansion and a passion to continue fighting the good fight for England.
The poem "If--" is a testament to growing up, and lends some advice to the reader. In terms of its biographical use, the line can lend a better understanding of the world Kipling was living: the Victorian period.

In the poem "If," Rudyard Kipling provides guidance in how to become a man, or more universally, how each person can truly live the live he or she has been given. Throughout the poem, Kipling speaks to an unnamed "you." At the end, he directs the words to "my son," but it's unclear whether he's speaking to his own biological son or a more general "son" of mankind. The advice throughout the poem is practical advice for anyone.
In order to understand this particular line, the reader must back up to the beginning of the stanza:

If you can make one heap of all your winnings
And risk it on one turn of pitch-and-toss,
And lose, and start again at your beginnings
And never breathe a word about your loss;

The speaker, who is presumably Kipling's own voice, imagines those situations in life when you risk it all. Another adage commonly used is "putting all your eggs in one basket." These are the moments when you think you know the direction to take, and you risk everything on this opportunity.
And then, sometimes, you lose.
Kipling says that when this happens, you should simply start back at the beginning and "never breathe a word about your loss." The loss isn't important. The courage to start again and to find another path or a different opportunity is what you should focus on.
From this, the reader can surmise that Kipling is a man of courage and perseverance, always ready to learn from his mistakes and always looking forward and not backward, moving on to the next opportunity in life.