Valve guides don't usually break but they can come loose. I've seen worn or loose guides use a lot of oil. It's possible to have a plugged vent on the valve cover but I really doubt this is the problem.
Don't bother with trying to replace the valve stem seals. It's only useful for a while and they will wear and start leaking quickly. Have a look at your spark plugs. The one/ones with heavy deposits is the cylinder you will be having problems with. Heavy oil consumption in a cylinder with normal compression will cause it to run hot and/or lean out which will burn up piston rings. This will score the cylinder walls.

Once you find which cylnder/s that are affected you may need to remove the head/s to have it repaired.