The S'forno (Vayikra 11, 2) explains that the commandment to build the Mishkan came as a direct result of the sin of the golden calf. Bnei Yisroel's situation at Matan Torah paralleled Adom Harishon's situation before he sinned: Hashem's presencecould be felt everywhere. However, after they sinned by making the golden calf they lost their ability to feel Hashem's presence, just as Adom lost that ability as a result of his wrongdoing. The Mishkan was meant to address this loss by serving as a dwelling place for the Shechina in this world. Additionally, Bnei Yisroel were thereafter prohibited from eating certain animals and creatures. Because they defiled their bodies with sin, they needed to remedy this defilement by purifying their bodies and refraining from specific foods.
Likewise, the S'forno writes (Bamidbar 15, 3; 20) that after Bnei Yisroel's debacle with the spies (who returned from scouting out the Land of Israel with a negative report), Hashem instituted additional mitzvos. They were commanded to bring flour and libation offerings as a supplement to their korbanos. Having distanced themselves from Hashem with their transgression, they were required to bring another sacrifice whereby they could once again feel His closeness. Furthermore, sin disconnects the transgressor from the Source of all blessings, therefore, Bnei Yisroel were subsequently commanded to separate challah from their dough and give it to a Kohen as a means of bringing blessing into their house.
Rav Wolbe (Shiurei Chumash) gleans two fundamental lessons from the S'forno's explanation. Firstly, every sin lowers the perpetrator one rung on the spiritual ladder, thereby forcing him to strive harder to gain the closeness to Hashem that was previously felt. After every sin, Bnei Yisroel needed yet another mitzva to enable them to reclaim their former spiritual status.
Additionally, we must take notice how the Torah guides a person on whatever level he finds himself. It is because the Torah is eternal, that it contains within it the guidance needed to direct each and every person. Each wrongdoing on Bnei Yisroel's part brought them down another notch in their spiritual standing; nevertheless, the Torah "stepped down" with them and added a mitzvah for their benefit. In whatever situation a person finds himself, or what his spiritual state of affairs might be, the Torah provides direction so that he will be able to live his life properly.
The Torah was given more than three thousand years ago to act as our guiding light, and it continues to shine brightly today to each person on his individual spiritual level. There is no reason for one to feel down or depressed about his spiritual position because invariably the Torah addresses his specific situation.