What are you trying to achieve?
Try making a 2D sketch on a plane (you could project your 3D Skechers onto it) and use the 'Split' tool.
you can split the part by using split tool using the inner line you have shown at the picture........
If u want to trim the Sketched area then....
1- take a projection plane above the surface,
2-sketch over the section which r required to be trimmed.
3- Trim the surface using Trim> remove selections.