Can't split surface

Hello, I’m having trouble splitting a ring shink at the top to make space for my setting with polylines. I have a book with me that tells me to use the split command to split the top and then delete the resulting section. It has not been working.

I’ve also been using boolean split and boolean difference by creating a triangle and then extruding it to make a closed polysurface but it is just joining itself onto the shank instead.

Both of the errors are pictured below. I’m not comfortable sharing the 3dm because my name is on the shank.

Could you make a copy of the shank with your name removed, and then post a .3dm file containing the shank without name and the cutting objects?

Try splitting from a frontal view. Failing that, try trimming from the same view