It doesn't happen often, but it happens. Many factors converge at one place at one time, and falling flat happens. There is no bounce, only a splat. Feelings, realities, reports, experiences, misunderstanding in a primary relationship can converge at one moment resulting in a downward 'splat'.
To understand this reality, one can research the elements of a sine wave. When a person utilizes the AC power that is readily available in the USA they are using 120 VAC. What is actually happening is an alternating sine wave with peaks up to near 170 volts and valleys down to 50 volts is being cycled at sixty cycles every second. It all happens so fast, we only think of this sine wave as 120 volts, but it is anything but a static level.
In human life, we tend to think statically instead of dynamically, and don't take into account the sine wave of life. These sine waves, or cycles, or seasons of life are filled with alternating highs and lows. In recovering from cancer/chemotherapy and/or any major issue one must take into account the cycle of one's life. We tend to only write about or hear about the averages or peaks of the human experience, but every living thing goes through these alternating experiences. The energy lulls, emotional blahs, mental fogs, relational misunderstandings, physical depletions, etc, are not the topics of best sellers but they are the realities of every human life, especially those of us overcoming cancer and its treatments. Add to this complexity the differing personality types comprised of a unique combination of personality traits these cycles of life are manifested in many differing ways. Add to this complexity the multiple side effects of the drugs and one can become overwhelmed with these cycles when they alternate to the lows way too often than one is accustomed to. How to handle life when one's cycle is on the bottom for an extended period of time is at best difficult. To sleep when sleepy, to rest when tired, to relax when stressed, retreating to rebuild a relationship, to build up oneself when torn down, to eat when hungry, to drink when thirsty, to seek God when lost, to not do stupid stuff when you don't know what to do: all of these are cycles of life that if ignored or denied keeps one at the bottom of the sine wave longer than necessary. Admitting to oneself, others and to God where one really is may just help the 'splat' experience to eventually cycle back up to some sense of normalcy. We are not voltages, and our cycles do not occur sixty times a second, but they occur. When and if one's main areas of enjoying life all cycle to the bottom at the same time and place is a dangerous time of life. It is at these moments when God's grace and other's grace manifested through the comfort and counsel of the Holy Spirit are so vital and necessary to cycle upwards to help one keep "walking through the valley of the shadow of death" (Psalm 23: 4).