It's a basic doctrine: Opposition in all things. We can't appreciate the sweet without knowing the bitter. Good can't exist without evil. Light is meaningless without dark.
In theory this is all well in good, but in practice it can be confusing and discouraging. Because besides the rule of opposition, I'm taught other basics as well such as wickedness never was happiness and the righteous are blessed for their righteous works. How do I reconcile these two truths? Especially when life seems wrought with unbearable trials despite all my righteous efforts. One such trial was post-partum depression which turned into a reoccurring challenge. When grappling with depression, I often feel a loss of agency. My ability to make choices, especially in regards to my emotions, diminishes.
In the midst of these feelings, I feel as out of control as someone who has surrendered their agency and positive emotion for drugs, alcohol, or other unrighteous behaviors that started with their own deliberate action. How is this fair? Why am I punished with this affliction when it's not a consequence of some evil doing?
The Mosiah in the Book of Mormon, there's another case of parallelism between those who made unwise choices (People of Zeniff) and those you are doing their best to be righteous (People of Alma).
Zeniff's people weren't terribly wicked, but overzealous and left themselves to be taken over by the Lamanites. They bore grievous burdens and were essentially slaves to the Lamanites.
On the other hand, Alma's people risk their lives to follow God, but also got captured by the Lamanites and forced to bear hard burdens.
Mormon tells us that Alma's people endure this because because "the Lord sees fit...to trieth their patience and their faith." Mosiah 23:21. The whole opposition idea that even the righteous have to endure hard things. But hey now, where are those blessings the righteous are promised?
By contrasting the difference between these two almost-identical situations, we see certain blessings that made the same trial more bearable for those who didn't bring it on themselves. First during their bondage, their "burdens were made light" (Mosiah 24:15). Which maybe doesn't mean easy, but means easier. Next, their methods of escape where similiar where both groups of people slipped away by night. However, Zeniff's people had to work a little harder and drug the Lamanites themselves, whereas the Lord took care of the drugging for the people of Alma, "[causing] a deep sleep to come upon the Lamanites."
In the midst of hardship it can be difficult to see that we are blessed. When we strive to obey God, trials are inherently easier because we don't have to relearn how to follow Him. Likewise even though depression can throw me off course, I still have my good habits and situations in my life that help me climb back out that darkness.
In the Book of Mormon, good and evil can seem so black and white, but we see that all people experience the same things, making us dependent on God's grace, but our righteous efforts are not in vain. They never are.