I have been wondering [but not actively doing research on] how the body responds to traumatic stuff. I'm not sure if it could be considered trauma at all but there was something that heavily bothered me. I tried to not think about it but whenever I felt safe or happy, it would just strike me really hard for no reason. causing some random spasm [or something like that]
I'm going to revel a bit and reveal one aspect of my personal life: my job is I'm a clinical psychologist.
Doing some armchair psychology on Usagi though, and taking into account Cosmos part 1 seems to be happening on a span of a few months, it could be Usagi didn't literally "forget" Mamoru turning to sand right in front of her. Rather, it seems more likely she unconsciously overruled that which she saw for a fantasy that Mamoru simply is far away - maybe made easier for the fact she fainted right after, so she could be thinking him turning to sand was something of a delusion. Of course, all of this happens involuntarily.
If she was a real person, it'd maybe be a bit more tricky to realize how much of this traumatic experience actually happened and how much of it is the person's defense mechanism for her grief. Naoko makes this a bit exaggerated by showing her emotionless eyes whenever someone mentions Mamoru.
If one doesn't have healthy ways to cope with loss (if the loss was too sudden/traumatic, if it's someone without close friends or family to talk to, if they're not emotionally strong to withstand the new routine without that person, aren't able/can't afford to go to therapy, etc.), ignoring the trauma trigger, intentionally or not, might help to soothe the anguish. Doesn't mean whatever issue was developed by trauma was "cured", as all it takes is some soft exposure to whatever triggers the trauma for the person to even feel physical pain (like when Seiya tries to remind Usagi of how Mamoru died).
Even me, myself, understand how this feels. When we lose someone and it's particularly painful, simply trying to stop thinking about this person is almost a survival strategy, because otherwise it becomse torture to do normal things like working, going out, exercize, etc. All it takes is accidentally seeing this lost one's pic because your friend posted it on an Instagram story and I can spend days having nightmares about it.
Thinking in therms of Usagi, I think as painful as it must've been for her to remember how Mamoru died, her confronting it is what ultimately gave her motivation to go after Galaxia and, eventually, make Mamoru come back to life. Again, if Usagi was a real person and her boyfriend died in front of her in a car crash or something, especially at an impressionable age as somenone just entering high school, I could totally understand how she'd prefer to pretend it didn't happen just because of how painful it can be.