It would have to not about remembering or knowing. It would be a sort of believing without having even the proof that you have inside. Sort of like if you have suddenly forgotten all the happy times during a breakup. In this situation, you actually have a similar thing going on, the depression overshadows the happy memories and you lose your head.
But you still believe in a chance for either other without remembering anything. Sort of "I wanted to be with her for a reason. I don't remember that now, but I believe this isn't her. She isn't this hateful person who wants me harm. I believe in her even if things feel tough now."
A sort of unconscious devotion. Reminding yourself doesn't work. Trying to convince yourself this is a dream doesn't work. Conditioning "works" but only by being completely stubborn and crazy (and I have a feeling this isn't how May works). It isn't about remembering, it's about her saying, "no matter how many tries, I won't give up on you." Despair is not defeated by memory, its opposite is hope. And love.
About this approach, this is an extremely bad idea. May is used to being alone, it wouldn't make her upset, not long term. She could cope. But May has Bern as her better half. She was cold and uncaring as a person, before she met Bern. This would be a regression, not a progression.
Also, it wouldn't work. If she removed one fear, another would take its place, and another and another. If phase 3 was ignoring things and moving forward, phase 3 would be like phase 2. Corona says that they were able to move "halfway" through phase 3, after 5 years. I do not believe this is actually the case. She has no way of knowing this without crossing into the next phase, does she? In fact, if it is truly despair, the area actually has NO space. Meaning if you tried to bluster through like Corona, the illusion would likely reappear again and again. If you killed your emotions, the illusion would disappear and then you would walk and walk forever (replacing emotional despair with limitless space until you give up). This is not something you walk past at all, this is something you overcome.
It may be hard without her memory, but she has to believe, even when she can't remember why.
Update: I guess if I had to imagine this I would look at it as two squares of paint next to each other. One is black, the other white. They can't turn the other one entirely black or white, but they can manage grey. The Dark Cell acting with despair on May is like black getting dumped on white. "The world sucks, politicians are evil, and there are people out there that want to hurt me. And I'll never be loved." This is usually how it plays out, because it is difficult to keep a positive attitude when you see depressing things all around you. But this is precisely what May has to do, to turn the black grey with her extreme whiteness. Kinda like this.
https://www.youtube.com/watch?v=uuC9yxJ6_pY