Here, the Torah refers to a specific scenario in which the flow of sustenance is reinstated. This is due to the significance of the mitzvah of Shemittah. Observing this mitzvah has the special power to bring forth Hashem's blessings. Even after His flow of bounty has been hindered by a lack of Bitachon, honouring this mitzvah resumes the flow of blessings. At the same time, it also teaches us a lesson in the meaning of Bitachon, and the detrimental effects of the lack of it.
The exception mentioned in regard to Shemittah does not apply to other instances, implying that our common assumption that concern about our livelihood is beneficial is, in fact, incorrect. In reality, this worry has a negative impact, actively disrupting the natural flow of sustenance towards us. Hashem, with His infinite mercy, has designed the world in a way that ensures we lack nothing. As a result, our sustenance is readily available to us by default, and we simply need to avoid blocking the flow of abundance through doubting His providence.
The Divrei Yisroel of Modzitz explains this concept, quoting his father Reb Shmuel Eliyahu of Zvohlin, who compared parnassah to a boat at the water's edge. He explained that just as a small push can set a boat adrift, even a slight worry or sadness has the potential to repel one's livelihood, causing it to drift out of reach, despite having been within grasp beforehand.
He further quotes his grandfather, Reb Yechezkel of Kuzhmir, who pointed out the absurdity of the people who worry for Hashem, but not for themselves. Meaning that despite Hashem being the ultimate provider, people often worry about their future sustenance, while neglecting to focus on their quest for Yiras Shamayim which, is the only thing that one must really work for on their own, as our Sages tell us (ברכות לג:): הַכֹּל בִּידֵי שָׁמַיִם חוּץ מִיִּרְאַת שָׁמַיִם - Everything [in this world] is in the hands of Heaven, except for the fear of Heaven.
Attaining Yiras Shamayim is dependant on our efforts, while our sustenance is totally up to Hashem.
(דברי ישראל בהר ד"ה או יאמר ונתנה הארץ)