The Gemara (פסחים קיח.) tells us קָשִׁין מְזוֹנוֹתָיו שֶׁל אָדָם כִּקְרִיעַת יַם סוּף, the sustenance of man is just as difficult as the splitting of the sea. Many meforshim ponder the meaning of this Gemara. Krias Yam Suf was indeed one of the most miraculous occurrences in the history of the world. However, we know, that absolutely nothing is actually "hard" for Hashem. Both the natural course of the world, including sustaining every living being, as well as the rare miracles that Hashem performs, are equally effortless to Him. Why is the word קָשִׁין - difficult used, where mans' sustenance is compared to Krias Yam Suf? And what exactly is the connection between the two?
Our Obligation to 'Go'
The Chofetz Chaim explains the similarity between these two facets of Hashem's running of the world. Before Krias Yam Suf, Klal Yisrael could not perceive a natural solution to their fatal predicament. Before them was the raging sea, behind them were the advancing Egyptian troops; there was no conceivable avenue of escape. However, Hashem instructed Moshe, דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְיִסָּעוּ - (יד טו) instruct Klal Yisrael to continue journeying ahead. Indeed, Nachshon ben Aminadav continued journeying straight ahead into the sea, and only when the waters reached his neck, did Hashem command Moshe to stretch out his hand over the sea, and it miraculously split. Hashem first commanded us to do something, to continue journeying on, and only after we did our part, did Hashem deem us deserving of His great miracles.
So too, says the Chofetz Chaim, does our sustenance come about in the same fashion. Hashem decreed that we make a token effort. And though the effort is completely unrelated to the ensuing results, we still need to do our part. However, while doing our required hishtadlus we must bear in mind that just like by the splitting of the sea Klal Yisrael could not fathom from where the salvation will come, so too, it is often difficult for us to understand how we will have ample sustenance given our meager efforts. Often it seems that the equation simply does not add up. It is not naturally possible for us to procure the required amount of money given our circumstances. Yet, we must realize that nothing is "natural", not Krias Yam Suf and not our sustenance. Nevertheless, it was mandated that we do our required hishtadlus, while bearing in mind that Hashem does not need our help, and our salvation can come from any direction.
Hishtadlus can be Hard
Therefore, the Gemara uses the word קָשִׁין. Because both by Krias Yam Suf and while trying to sustain our families, we need to start by doing a natural act of hishtadlus. Because this is 'natural', therefore it is קָשֶׁה, difficult. Since our part must be according to the rules of nature, therefore it is difficult. However, we must remember that in truth the results are always effortless for Hashem. It is only our required efforts that are often difficult.
By Krias Yam Suf Hashem commanded us, דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְיִסָּעוּ - Speak to the children of Yisrael and let them travel. So too Chazal (תנא דבי אליהו רבה פרק יד) caution us בִּטְלֵיכֶם בִּטֵּל, a person may think that because Hashem is In Charge, and He does not need our 'help', therefore we can sit and idle. However, תַּלְמוּד לוֹמַר וּבֵרַכְתִּיךָ בְּכֹל אֲשֶׁר תַּעֲשֶׂה the possuk teaches us that Hashem will bless you in all that you do. We cannot sit back and await miracles. We must do a minimum amount of hishtadlus, while constantly staying aware that our salvation can come from anywhere. We need to "continue traveling" and thereafter we may merit our personal Krias Yam Suf. For both are equally easy for Hashem.
According to the Chofetz Chaim this saying about the difficulty of kriyas Yam Suf and parnasah refers to the receivers; just as it was difficult for the Yidden at that time to "journey on" without understanding how they will be saved, so too it can be extremely trying for a person seeking sustenance, as he must do the effort that is within his reach while recognizing that the results are completely in the Hand of Hashem and may be totally unfathomable.
(חפץ חיים על התורה פר' בשלח בהג' מעשי למלך)
No Natural Resolution
Many other seforim also expound on this connection, with an added emphasis on the fact that הַרְבֵּה דְּרָכִים לַמָּקוֹם, Hashem has myriad ways in sending us our salvation. One should not be discouraged by the difficulty inherent in his hishtadlus, and by the seeming impossibility of reality. The יִשְׂמַח יִשְׂרָאֵל quotes the Rabbi R' Bunim of Peshischa, who references the unlikelihood of a sea splitting. He says that by Krias Yam Suf the situation seemed dire, and jumping into the sea seemed like an act of futility, therefore Klal Yisrael did not see from where their salvation would come. So too, regarding parnassah, as well as other areas of challenge, salvation comes in ways one could have never anticipated. The sea can suddenly split, and then a path to safety materializes.
He adds that the same is true in the spiritual realm. Often one is mired in challenges and he does not see how he can extract himself from the quicksand of spiritual difficulty. But especially when he has reached the point when he cannot understand how he will ascend again, that is when the Heavenly salvation can come in the most unexpected mannaer.
(ישמח ישראל שבת פסח אות קלח)
When one strengthens himself with this mindset, he lives a life of tranquility. He knows that he does not need to see a realistic resolution to his difficulties, he simply needs to fulfill the requirement of hishtadlus, and rely that Hashem's salvation can come from any direction.