What does "force your heart" mean?

English is not my native language and I don’t fully understand what does the expression "force your heart" mean.

I have seen it in the Rudyard Kipling’s poem If:

If you can force your heart and nerve and sinew

This is one of my favourite poems, but you need to read the whole poem (see below) to really get the meaning. The poem is about how one should cope with life's trials and tribulations. In this particular line 'nerve and sinew' refers to body and soul. When you feel you are defeated by the harshness of a situation, you are tired and lack the willpower then dig deep, hold on and you will find the strength and determination to carry on.

If you can keep your head when all about you
Are losing theirs and blaming it on you,
If you can trust yourself when all men doubt you,
But make allowance for their doubting too;
If you can wait and not be tired by waiting,
Or being lied about, don’t deal in lies,
Or being hated, don’t give way to hating,
And yet don’t look too good, nor talk too wise:

If you can dream—and not make dreams your master;
If you can think—and not make thoughts your aim;
If you can meet with Triumph and Disaster
And treat those two impostors just the same;
If you can bear to hear the truth you’ve spoken
Twisted by knaves to make a trap for fools,
Or watch the things you gave your life to, broken,
And stoop and build ’em up with worn-out tools:

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

If you can talk with crowds and keep your virtue,
Or walk with Kings—nor lose the common touch,
If neither foes nor loving friends can hurt you,
If all men count with you, but none too much;
If you can fill the unforgiving minute
With sixty seconds’ worth of distance run,
Yours is the Earth and everything that’s in it,
And—which is more—you’ll be a Man, my son!

I think it's worth pointing out that Kipling is (probably) not literally talking about your heart organ, but rather "heart" used as an expression for determination. "heart" is commonly used in this way, for example saying that a sports team "doesn't have the heart to win the championship". Obviously they have hearts in their chests, but the speaker is saying they will give up at some point when it gets too difficult.

There are a lot of examples of this (organs meaning something else) in English:

"heart" can mean "determination"

"nerve" can mean "the ability to stay focussed and courageous in the face of opposition"

"guts" can mean "bravery"

"stomach" can mean "the ability to do or view something without being too disgusted to carry on"

"gall" can mean "the ability to ignore the fact that what you are doing might offend people", or more simply, "impertinence"

etc.

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

He's saying that if you can overcome your human frailties by force of will, endure things when you should be past the point of human endurance....

It was very much part of Kipling's view of "Britishness" that a man should be stoic and resilient. The whole "Stiff Upper Lip" culture is ingrained quite deeply into us Brits and was certainly how I was raised.