@Bill D Perhaps because the shape is complex, if you cut it in half, cut one side, model it, and then mirror it, it may not become a solid. I made only one side without cutting, and finally made the whole thing symmetrical, and it worked fine.

@Peter kreDosy Thank you for the addition.

@Nikita Kapustin >If you want to use "standart" fillets you can add variable points and increase the fillet in the yellow area. This is the method I used at the beginning of the helmet course. As you said, it can be applied to this part. I'll try.Thank you Nikita.