Perhaps the issue here is that you are unfamiliar with human cognition.
Perhaps you cannot will yourself into a state of believe--or believe that you do not, but the fact is that people do choose to believe all sorts of things.
People believe all sorts of things, but they don't choose to believe it.
The presence of religion should be sufficient example. Or one partner chooses to believe the other when they are told that the partner was working late, was faithful, that it didn't mean a thing. Or that the check is in the mail. I'll do it next Thursday. All sorts of things. Or that this candidate is more likely to do a good job serving in their chosen office and representing viewpoints that I agree with. People choose to believe all sorts of things are true, even in the face of plain evidence to the contrary. Very obvious case in point: Trump supporters/anything at all that Trump says.
People believe strange and fantastic things, but that does not mean they did it by an act of will.
A victim is traumatized. Trauma can do all sorts of things: it can change emotional affect by increasing it--or by making it very flat. It can cause lapses in judgment, lapses in memory, altered perception, fragmented memory, heightened sense of fear or alarm and many others.
What is the relevance of the above paragraph?
A person presents themselves as a victim. No matter how skeptical one is or has reason to be, if one is actually interested in the truth, the best course of action is to believe the victim---and then investigate the facts as best as can be done. If the victim is straight up maliciously lying, you are much more likely to get pertinent information and cooperation--and to get them to let enough guard down to ferret out the truth if your initial stance is belief. If the victim is traumatized to any extent at all, belief in their statement allows them to trust themselves and investigators enough to aid investigation rather than hinder it. Even if the account is highly unlikely, it almost certainly contains some elements of truth. The only way to get at the truth is to (initially) take the account at face value. A drunk claims that Michael Jackson mugged them in an alley. Obviously that's untrue: Jackson is dead. One could either dismiss the drunk as a lying drunk--or investigate, believing that something happened involving Michael Jackson--and discover that there was a Michael Jackson impersonator performing that night and indeed, they went into the alley between sets and rolled the drunk and went back on to perform their next set after their costume change. But if you start with: lying drunk, you never find out anything.
You can take allegations seriously (that is, do your job of investigating) without believing somebody. For example, if somebody told me Michael Jackson mugged them in an alley, I would plain not believe them, because as you say, it is impossible. But that doesn't mean you couldn't investigate.
But I can't will myself to believe Michael Jackson mugged anybody last night, and neither can you.